[169indy]

I am not sure if your state plate has a reflectorized coating as the 'white' background (1970)?
Washington plates went reflectorized- white background in 68. When I tried to 'clean up' the yellowish aged plates no real chemical or mechanical means, was found that effectively produced great results.

P.S. The biggest problem here is cleaning off the MOSS & Slime from the plates, it is amazing we do not have barnacles on em!