Early US Nuclear Weapons Designations.

[RyanC]

History of Project A by NF Ramsey

Implosion Bombs

Model 1222 "Fat Man": Early design in which the H.E. core was held together with an inner dural shell consisting of 12 pentagon sections surrounded by an armor steel shell consisting of 20 triangles. Required 1,500 bolts to assemble.

Model 1561 "Fat Man": Later design which adopted the familiar shape we know; and much simplified assembly.

F13: Dummy Model 1561 "Fat Man" unit dropped 1 August 1945.
F18: Dummy Model 1561 "Fat Man" unit dropped 5 August 1945.
F31: Active Model 1561 "Fat Man" unit tested over Nagasaki 9 August 1945.
F33: Dummy Model 1561 "Fat Man" unit dropped 8 August 1945.
F101: Empty Model 1561 "Fat Man" unit assembled and held in readiness in the week following Nagasaki mission.
F102: Empty Model 1561 "Fat Man" unit assembled and held in readiness in the week following Nagasaki mission.
F103: Empty Model 1561 "Fat Man" unit assembled and held in readiness in the week following Nagasaki mission.

Gun-Type Bombs

L1: Dummy Mark I "Little Boy" unit dropped 23 July 1945.
L2: Dummy Mark I "Little Boy" unit dropped 24 July 1945.
L5: Dummy Mark I "Little Boy" unit dropped 25 July 1945.
L6: Dummy Mark I "Little Boy" unit dropped 31 July 1945.
L11: Active Mark I "Little Boy" unit tested over Hiroshima 6 August 1945.

What the "Dummy" tests consisted of was assembling the bomb; but instead of loading live nuclear cores of fissile material; they put in placeholders; and then dropped them over the ocean near Tinian; to test whether the various electronics of the bombs worked; like radar altimeter fuzes, the firing units, etc.

 

[RyanC]

I've been doing some study of documents on DOE's webserver, and I think the way WWII nuclear devices were designated was:

Physics Package (the stuff that goes boom):

Because Captain W.S. Parsons, USN was in charge of the ordnance design for both types of bombs from 1943-44 (he was removed from the Fat Man ordnance team in 1944, but continued on the Little Boy team), the physics package got the USN terminology of "Mark", e.g:

Mark I Little Boy
Mark II Thin Man (Plutonium Gun Bomb)
Mark III Fat Man

This is backed up by 95% bomb (dummy bombs having everything but the fissile) drop logs that reference:

Sphere:

2.5: Blocks, Composition of: MK III Plaster
2.7: Detonators: MK III Switch with small Det. Charge (8 each)
2.8: Detonator Retainer Type: MK III Switch

Overall Bomb Designation Number

The "model" number for the bombs are actually the blueprint sheets used by Los Alamos (Site Y) for configuration control; for in the same reference that provided the above MARK III wording:

TYPE OF TAIL: STANDARD: DRAWING NO. Y-1560-3

Y for Site Y (Los Alamos) is what I think Y means.

A report dated 12 July 1945 on DOE's OpenNet titled A Report on Kingman Program, October 1944 to May 1945

Has the following configurations listed:

LB-1418
LB-1491
LB-1791
LB-1850

FM-F10
FM-1222
FM-1291
FM-1560

I think post-war, the "Y Model" number was depreciated away in favor of the Physics Package providing the number, once things got away from the "wild west" of nuclear weaponeering.