not sure what you were asking about the L M and H?
but the L M H referes to the offset low medium high

I am having a similar problem the rims on my 65 are 15's with 6 lugs and 3 3/4 back space. want to move up to 17" but closest thing I can find is 4.5 back space and every wheel I find is "U"gly.. I ws told in my case I needed a low offset and that range was 0-12mm. did nto ask about medium or high