I think that there is a interesting tip, EX-51...
Although not directly PASGT-related, a bit of background on those earlier experimental helmets may be of interest. The EX 51-1 was an evolution of the EX 49-3 prototype produced at Fort Benning - itself a refinement of the EX 49-1 (which sprang from the commerical Doron Crash Helmet).
A major issue was finally concluding as to whether a universal, "all-purpose helmet" was actually feasible or if armoured crews in particular needed a specialized type. In 1944-45 alone, the Armored Board at Fort Knox tested various potential helmet types for tankers, including (excuse the random hyphens) the T-10, T-12, T-13, T-16, T19E1, T19E2, T-20, T20E1. Between 1947 and 1952, the Army Field Forces Board tested (with report dates in parentheses) the Doron Crash Helmet (Dec 1947), EX 49-1 (Aug 49), Ex 49-3 (Mar/Apr 50), EX 51-1 (Oct 52).
In the quoted sources (below), the nomenclature shifts about. In the first source, the designation is given with a space but without a hyphen. In the second quoted source, no there is no space at all. In both cases, the official description is given in lower case. But, in captions it is upper case - eg: Liner, Helmet, Nylon Ex 49-1 (and note that lower-case 'x'!).
A Tanker's Uniform for a Tanker's Duties: A Research Report, Prepared at The Armored School, Fort Knox, Kentucky, 1951-1952 [April 1952]; pp 32-33
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https://mcoecbamcoepwprd01.blob.cor...rs/ASTUP/A-F/Committee 32 Tankers Uniform.pdf
"At a meeting in the Metropolitan Museum of Art in New York City in January 1949,a universal helmet design was studied in some detail and the liner, helmet, nylon, EX 49-1 evolved as a possible universal helmet [...] This helmet was constructed of seven-ply laminated nylon fabric. It offered considerably mere coverage to the back of the head and ears than the standard M-1 helmet. It was intended for use without an external shell by crew members of armored combat vehicles. The suspension was similar to the standard M-1 suspension. After testing, it was recommended that no further consideration be given to the liner, helmet, EX 49-1 for use by armored vehicle crewmen and no further development of a universal helmet be undertaken. It was recommended that a special helmet be developed for crew members of armored combat vehicles. 12
However, the idea a universal helmet was not to be defeated so easily. The Metropolitan Museum of Art developed the liner, helmet, nylon, EX 49-3 (Methacrylate Prototype) in can effort to correct the deficiencies of the liner, helmet, nylon, EX 49-1. This helmet was made of methacrylate merely for testing purposes. The liner, if adopted and produced, would be made of nylon.
At the present time the helmet, EX 51-1, a modification of the EX 49-3, is under test. This project has been assigned. an "A"priority. No results are obtainable at this writing. The EX 51-1 represents the latest development in helmets for armored vehicle crewmen. It consists of three parts; the shell, the liner, and the suspension. The shell is fabricated of heat treated aluminum alloy and is shaped to cover the head, fore-head, and ears. It is cut out at the rear (nape of the neck) to allow the wearer to turn his face upward without interference from the shell. It is also cut out at the front (forehead) to allow the wearer to use optical instruments. The hinged latches on either side of the shell not only provide fastenings for the chin strap but also give a means of locking the liner to the shell. The canvas chin strap has a quick release device at one end. There are two size shells. The small size weighs 1.33 pounds; the large size weighs 1.45 pounds. The liner is designed to give protection against small arms fire and shell fragments. It is fabricated of nine-ply, 2 x 2 basket weave nylon with the same shape and configuration as the shell. The liner is made to fit snugly into the shell and is locked into the sides of the liner to take either of two types of suspension, M1 or M5. There are two sizes of liners: the mall size weight 1.39 pounds, the large size weighs 1.51 pounds."
Technical Memorandum 4-73
Historical Documentation of the Infantry Helmet Research and Development: USAMC Five-Year Personnel Armor System; Technical Plan; Charles W. Houff & Joseph P. Delaney; February 1973; pp-8-9
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https://apps.dtic.mil/sti/pdfs/AD0908190.pdf
"T21 shell had a curvature in all directions at all points in the helmet, established through anthropometric studies of the human head and purported to decrease size with no sacrifice of area coverage, yet increase strength and protection. It weighed two pounds, three ounces and was worn with the standard liner. The T22 was smaller than the T21 and was a one-piece helmet, worn without a liner. The T23 was larger than the T21 and incorporated a thicker liner. [...]
The T24 helmet had an aluminum shell modeled after the M1, with a laminated nylon liner. The T21 E1 utilized the nylon and aluminum but was based on the contour pattern of the T21. [...]
The period 1947-1951 saw research continuing on helmet and. liner designs and on new material. The T21 was modified according to test report comments and became the T21E2, having additional coverage at the nape of the neck. However, with the additional coverage, weight increased to that of the M1. In January 1949, a decision was made to suspend development of the T21E2 and the non-ballistic tankers helmet and to concentrate on an all-purpose helmet. The first model was the EX49-3, which became the EX51 after test and modification. The EX51-1 was two-piece, having an aluminum shell and a 9-ply nylon liner. It had two sizes, small, to size 7 1/8, and large, size 7 and larger. The EX51-1 utilized the M1 suspension and weighed under three pounds. It was extensively tested [...].
Tests by Army Field Force Board No.3 in 1952 concluded that the EX51-1 was unsuitable for the Army Field Force, pointing out that it exposed a larger area of the head to missiles, impaired hearing and interfered with communication equipment. Moreover, the hardware attachments were both fragile and difficult to operate. In this report, Board No.3 stated that it had commented favorably in 1946 on the T21E2 and the Doron Type II and concluded that they were suitable for further development. The Board questioned the soundness of an "all-purpose helmet" (319).
An Army Helmet Conference at the Office of the Quartermaster General in Washington, D.C., 9-10 December 1952, decided to discontinue the all-purpose helmet and require two helmets -- one infantry and one combat vehicle crewmen --and developed the military characteristics for these two helmets.
In 1953 the Combat Helmet T53-2 and T54-1 Helmet Liner were engineering-development (ED) tested 'and engineering service test (EST) quantities of these items were produced for testing in 1955. The T53-2 was a 35-ounce aluminum shell having a 15-ounce nylon liner. It increased the protected area by 10 percent, provided an improved suspension system, offered better ballistic protection and was considered to be more compatible with the armor vest than the M1."
