DutchPlateSpotter It looks a bit like this Oklahoma one:
Looks similar yes, I'm unsure if it's Oklahoma though. If it helps anyone else, I've singled-out the sticker here, though the quality is far from perfect. Looks to be valid between 2017-2018?
I looked into inspection stickers from Washington D.C., Virginia and Maryland. It doesn't look like a Virginia and Washington D-C. one, moreover Maryland doesn't issue inspection stickers, as they have some different system. I don't know the state which issued this sticker. I looked at pictures of every states inspection stickers and the Oklahoma one came closest to this one. I hope you find the exact same one eventually!
DutchPlateSpotter I looked into inspection stickers from Washington D.C., Virginia and Maryland. It doesn't look like a Virginia and Washington D-C. one, moreover Maryland doesn't issue inspection stickers, as they have some different system. I don't know the state which issued this sticker. I looked at pictures of every states inspection stickers and the Oklahoma one came closest to this one. I hope you find the exact same one eventually!
Thanks for doing the research behind this sticker. I've tried searching too, even using vague terms like 'Green Inspection Sticker', to no avail. Most states which require inspection stickers, usually have the stickers in the windscreen. I saw loads of US-embassy cars with Virginia stickers in the windscreens whilst in Berlin. I can't think of many states which require stickers to be placed on the bumper though, Hawaii is an exception to that, but this ain't from Hawaii.