So this thought came to me from a throwaway line in Sidney Sheldon’s Master Of The Game about how the main character’s conglomerate started manufacturing tanks in World War I (along with other war material). How hard is it to slip a tank company into an alternate history?
There’s two boring solutions. One is that it’s easy if the story calls for it, with a focus on armored vehicle economics not usually being beneficial to a book (especially a Sidney Sheldon one). Another is that they can, especially during the World Wars, be just a contractor that built tanks designed by someone else (see a lot of railroad locomotive plants in World War II). A third is that they end up as the main winner for a gigantic wartime or Cold War contract and just become what General Dynamics Land Systems (to give one example) is in real life. A fourth is if severe politics (read-no reliable import partners) are involved.
But privately designed tanks for private sales? That’s tricky. There’s really only a few windows, the interwar and middle Cold War periods. Otherwise, you just have a glut of WWII surplus/early Cold War military aid or an equally huge one of advanced technology/later Cold War surplus.
And even then, for every success like the Vickers MBT, you have failures like the AMX-40 and Osorio, to say nothing of one-customer wonders like the Stingray. Both political power and economies of scale are tough to overcome. Yet there’s always the chance of getting an export order and then having the exported tanks do well enough to trigger more interested customers. It still isn’t going to come close to the T-55 or Patton, but it can work.