Wasn't there a rule that to be considered a "production car" and thusly eligible for some races that they must produce 400 cars... The 401st added was the one-off Chevy engined goose built for Bill Mitchell.
I know the Miura was originally planned to only build 400, hence the name P400 (Produzione 400). Granted they built much more once sales took off...
I was wrong once in the 70's, so I'm overdue for a potential error... So who knows...
MH