Hi and welcome to CT This issue had no MM from Philly mint see the comments section https://en.numista.com/catalogue/pieces55.html
I thought all 1988 quarters had mint marks. You are saying some don't. I've never seen another. been at it since 1950
Thats what I found. Started with th P mint mark in 1980 and ran to the end of the series in1998 and back for 2021. The quarter I show has no visible mint and is not reconized by NGC or PCGS as an error coin. Jim
Most likely just a filled die. There is also a fair amount of circulation damage in the area, if there was just a trace of the P it might be obliterated.
It is probably a grease filled mm - it doesn't add too much value, especially in the coins condition, unless you sell it on etsy. There are litterally hundreds of "NO MINTMARK QUARTER 1983!!!!!" with a starting price of $400+. But yeah, I honestly wouldn't pay more than face value for that. Keep searching.