44 AMP
Check the safety, primarily where the ridges contact the the engagement surfaces of the firing pin. The ridges are not that thick or pronounced, and coupled with repeated usage and possible metal fatigue, could cause the safety to fail. Also check the slide top cover for fit and tension. The forward retaining arms of the top cover can weaken over time, causing the top cover to fly off the gun, along with the rear sight, firing pin lock, spring, and retainer.
The AC42 was produced in fairly large numbers, some 120,000 being made in that year. Beginning in October, 1940, Walther began using a different serial numbering sytem with their pistols. The first 10,000 block of pistols would be a no letter block; the next block would start the numbering over again, only now the letter "a" would be added as a suffix after the serial number. The next block would then use the letter "b", and so on. This would continue until the beginning of the next year, at which time the factory date code would be AC41, and would start a new no letter block of serial numbers.