1876-S 50c Variety Check

Also, is there anything special going on with this MM or is it PMD? If it's not PMD, how would the aforementioned letters/numbers change?

The WB-102 comes from Randy Wiley and Bill Bugert's original book "The Complete Guide to Liberty Seated half dollars". The WB-24 that you referenced above is from Bill Bugert's newer "A Register of Liberty Seated half dollar Varieties, Volume I, San Francisco Branch Mint".
The WB-### was the original way they referenced die marriages and only consisted of WB-101, 102 and 103. In the newer versions there are 46 different die marriages which now consists of 18-WB-101, 26-WB-102 & 2-WB-103 die marriages.
The coin you are showing is a WB-102 and more specifically WB-37 and not a WB-24. It has a very small S mintmark centered below the crotch and either recut up or recut over a micro S mintmark, which your image shows as well.

For Randy Wiley and Bill Bugert's original book "The Complete Guide to Liberty Seated half dollars", you can find it online here - https://archive.org/details/compgui...lawr/page/n0 or if you would like a hard bound book you can sometimes find them on ebay.
For Bill Bugert's newer "A Register of Liberty Seated half dollar Varieties" you can find them here - http://lsccweb.org/BillBugertBooks.php and most are available for free download at bottom of page.