The actual vehicle in which they met their demise is located in a casino somewhere in Nevada. The holy grail of license plate collectors would be to own one of the numerous stolen license plates recovered from the trunk of their vehicle and used during their crime sprees to avoid recognition.
As many have indicated, police vehicles of that era were plainly marked......Detroit PD for instance had plain lettering on the doors to indicate it was a police vehicle.
In 1947, Michigan issued pairs for both passenger and truck plates. If you're trying to find some, join the "Michigan License Plate Collectors" Facebook group as it is filled with Michigan License Plate collectors who have just about everything for sale.
Join the "Michigan License Plate Collectors" Facebook group. Solely dedicated to Michigan plates, all are Michigan collectors with an endless supply of plates.
I'd opt to have it restored as well....or find one in with original paint suitable to your needs. I have had numerous plates restored, and they turn out great if you use the right restorer.
Join the "License Plate Collectors" Facebook group. Members are all collectors and I'm sure you'll be able to find an affordable plate for your display.