I agree with the spooning and the "about the same size". Can you post a picture of it with a normal quarter for comparison?
As a longtime former resident of Atlantic City, I can quite agree that it's not a slot quarter. The rim is far too sharp for that. However - and this should be a stipulation before even considering what it *might* be - it's not possible for the Mint to strike this coin without reeding. So, regardless of how the coin came to lack the reeding, it didn't happen at the Mint. They don't have a non-reeded collar of the correct size in the building. That narrows down the possibilities somewhat.