reubent,
Great question! I've looked into it. The answers I received were basically, time and experience.
I know Fremer is very knowledgeable here. You'd have to have a full time job in deciphering the dead wax/meanings to be an expert. He is the guru.
Regarding my mentioning of the Black Dub lp. I know it's a first pressing by the very fact that I ordered it the day I heard it was released. Since then, I know by my ongoing (looking) that it has be re-issued once domestically and Music On Vinyl has their version. (Most MOV are digital files, mostly sounding very good). I've had great luck with MOV, but, I think my pressing would be desirable?
From my meager understanding, regarding pressings I say are 1st or close... early pressings of say 60's/70's have in their dead wax on the matrix #'s ..for example 1A, 4A, .... or sometimes the letter/number can be reversed.... for instance B4= side 2...4 stamper/mother. I'm no expert, but this should give you some idea.
I'm still learning. Here is one I'd like deciphered...
My favorite lp ever, "Bookends", the copy I most listen to is..
Side one...XSM 11887 1F
Side two...XSM 11888 1B
I found this one for $1.00 at a church yard sale.
We are all learning.
Happy Listening!