According to Tom Batha in U.S. Martial .22RF Rifles, the first 6,000 were issued to the US Marine Corps in 1943. I've never seen a reliable estimate of how many were made. I own one as well (serial number 3721) and really enjoy shooting it - you can feel the bolt cycling inside the receiver, and it's accurate and reliable even with bulk pack ammunition. It's also the only semi-automatic .22 trainer used by US forces during World War II. I've attached a photo recently posted on rimfirecentral.com showing the Reising 65 in use.