The following might be of interest.
Faced with the task of sprucing up the Bloch MB 155 with the new, powerful and unproven Gnome et Rhône 14R engine, a young engineer by the name of Lucien Servanty (1909-75), future chief designer of the SNCASO SO.9000 Trident and SO.9050 Trident II / Trident III rocket-powered interceptors (8 or so aircraft built) and major player in the design of Concorde, quickly realised that a simple adaptation using the same airframe, a version known as the MB 156, would not take advantage of the potential of the new engine.
Servanty thus undertook a thorough redesign of the airframe, with an larger wing and various improvements. The prototype of the MB 157, incomplete in June 1940, was seized by the Germans as it was being evacuated to the southern part of France. Oddly enough, the German authorities eventually allowed development to continue, under their control, in Bordeaux.
Completed in early 1942, that prototype flew around March or April. Its pilot may well have been Bloch test pilot Zacharie Heu (1890-1953).
The aircraft was later moved to Orly, near Paris, where it was (partly?) disassembled and stored. It was eventually destroyed during an Allied bombing raid, in 1943 or 1944 I cannot say.
The very high speed of the aircraft might have been mentioned for the first time in a publication in an April 1954 issue of the bimonthly Aviation Magazine (
https://gallica.bnf.fr/ark:/12148/bpt6k5322336j --- p. 15, col. 3).
According to
http://sam40.fr/les-derniers-chasseurs-bloch-157-100-1011-1040-entre-mythe-et-realite/, that speed seemingly made its appearance no later than 1943, in a company brochure, as a predicted or expected speed (441 mph at 25,750 ft), under some sort of wartime emergency power condition - and not as an actual speed achieved during testing.
It is worth noting that the aircraft was then referred to as the SO 157. Marcel Bloch, who was Jewish, was no longer an acceptable chief designer. Indeed, he had been interned / arrested by the Vichy government in October 1940.