The lenght of the base fuselage was still the same. The difference was the ending of the fuselage that was, let's say, of the blunt type for the LeO-45 prototype but slightly longer and more pointy with the nav light for the serial LeO-451. Therefore there is the difference of 36cm in the total lenght of the fuselage. None of the LeO-45 references says whether the LeO-451-1 had the fusealage ending of the late shape. The pic of the LeO-45-1 in 1937 shows the blunt type ending there. Because the LeO-451-1 was the LeO-45-1 with the G-R engines and the Mercier's cowlings attached, the ending still might be of the blunt type as well . Anyway the first serial LeO-541 and all known planes of the earlier batch present the later variant of the ending.
Regarding the HS 404 gun ... as memo serves the French factories didn't supply complete planes to the Armee de l'air. Just they delivered the planes only but all the military stuff was attached by the military equipment unit(s) before sending to the squadrons. Therefore most of the factory prototypes were unarmed often.