Ghengis and chums,
This is something which I can speak from a knowledge base on as i'm a 20th C Military historian by profession!
You're absolutely correct, we did of course have interrupter gear on aircraft during WW2. But perhaps you may not know that it went back to WW1 - namely 1915. During WW1, the development of fighting aircraft was huge. Triple Alliance and triple Entente each developing their technologies to be one up on the enemy.
Germany were the first to use interrupter gear. In May 1915, the Germans produced a new and fast Fokker which was equipped with the new interrupter gear which enabled the gun to fire through the orbit of the revolving prop without the risk of hitting the blades. The Germans were therefore able to inflict heavy casualties on Allied aircraft and regained air superiority.
If in doubt, ask a specialist historian
...you may all be able to out gun me on the Private Flying forum...
but I can come into my own on this one
GQ