Much of it is anecdotal; at the time the records were made, consumers weren’t really focused on first pressings as such. Now, trying to collect older pressings, you need to rely on a combination of indicia, not all of which is described in one place. That includes the deadwax inscriptions, covers, labels, inners and record plant markings. First pressing from what country? And don’t assume that a first pressing is necessarily the best sounding.
Sometimes, it’s a fluke. For example, I found that a later MCA pressing of Lynyrd Skynyrd’s first album sounds better than my early SOS label (or the MoFi reissue for that matter). Also, on Free’s Tons of Sobs, it is commonly assumed that A2/B2 is a first press, but I’ve seen A1/B1’s offered, and sometimes, those aren’t on the earliest labels- the Island pink label "orange bullseye."
Go figure.
For some US records, multiple lacquers are cut at the same time, to provide numerous pressing plants with sufficient metal parts to press large numbers of records. Thus some indicia as a 1, 2,3 in the deadwax may not be indicative of earlier or later, but are essentially simultaneous.
