I thought this was a 45 cartridge. Buyer tells me it's 9mm. I know WCC made 9mm ammo but did they do it during WW2? Did America need 9mm ammo?

Seems like WCC did make 9mm cartridges during WW2. 

 

https://forum.cartridgecollectors.org/t/us-9mm-production-in-1943-a-proposal-for-steel-cases/25650

 

And this picture suggests it is a 9mm cartridge.  https://discover.hubpages.com/health/9mm-Parabellum-vs-45-ACP