I just finished registering 1940 plates that came with my 1940 Ford Deluxe Sedan. Thankfully, the plates weren't registered. It took them about 4 months to finally 'approve' of it and send me the stickers. I took the new plates and the old plates down to a local DMV (Fullerton, CA), and told them I'd like to switch the plates to YOM. They took a look at the 1940 plates (they make sure they are the real deal, not trashed, and in good condition), then they took photocopies of the 1940 plates. They kept the modern plates I had, and with the paperwork and photocopies of the new plates (and a registration fee), they sent it in to Sacramento. Four months later (and me getting worried that something happened as even the DMV website said 'processing' still), I got the title/registration with the 1940 plates added. They also sent the metal 'tabs' that you can put on the rear of the 1940s plates to hold the month and year stickers, which was nice.
One thing to note... California 1940 and 1941 plates exist, but 1942, 43, and 44, plates do not. For 1942 they used a orange 'add on' bar that was placed over the 1941 plate top to make it a 1942. In 1943, they added a small white tab in the top right corner with a V for victory. In 1944, with metal/steel in major need for the war effort, they kept the 1941 plate with the 1942 top tab and the 1943 corner tab and just added a sticker to the window. In 1945, they went back to plates, but only ONE plate. There are no matching 1945 plates (and it's the only year, I believe, that only requires one plate, not a matching two).
1940 Plate:
1941 Plate:
1942 - A 1941 plate with the top orange tab:
1943 - A 1941 plate with the top orange tab and a white corner V for victory tab:
1944 - A 1941 plate with the top orange tab and a white corner V for Victory tab with a 1944 window sticker:
1945 Plate: