Years ago the US Military used vehicle plates/tags made locally out of some sort of metal, with either painted on letters/numbers or vinyl letters/numbers. Sometime in the late 80's we started getting real pressed tags for the GOV's. While MP could be Military Police, it would most likely be just a random combo of letters. The vehicles were already marked MP or SP so there was no need to do a plate that way.
Here's one that sold on E-bay at some point.
Vintage 1940's 50's US Army License Plate | eBay
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