I can’t see any evidence of this being “key over crown,” it just looks like a plain key to me (maybe a close-up of the obverse key would clarify?).
 
	I don’t know of any web resource, you might have to start buying a book or two. Here’s the mint marks for James I taken from the SCBC,  which would be a good start! You can buy this for around £30 + postage from a dealer on here if you want one?
 
	https://rpcoins.co.uk/collections/catalogues-s-z/products/coin-of-england-2025-pre-decimal