With the mask ROMs themselves, you gotta remember that they were expensive to make, both back then and even more so now as the technology has fallen out of favor. When you have multiple different product platforms to release games on (MVS, AES, JPN, USA, EUR), you can't afford to waste money on different batches of mask ROMs for the same title. It wouldn't be a stretch to say that the Mask ROMs were the most expensive part of the average Neo Geo game either. Arcade games usually store all data for multiple regions anyway. It's most likely for this reason why SNK would release a game on one master batch of Mask ROMs and split them all up between the different sub-product lines. It's one example of why there are so many Metal Slug 1 MVS carts and so few on AES. 1996 was part of those dark years for SNK when they had a lot less money, having just stepped away from the financial loss on the Neo Geo CD. AES was kept probably because there was still enough interest from fans to make each release profitable, but not enough to warrant many AES carts of games like Metal Slug when it was doing so well in the arcades. Despite current values of AES Metal Slug X to 5 today, they were produced in very large quantities, for twilight years AES standards at least (that is to say, still low but not ridiculously low).
Long story short, Neo Geo mask ROMs always have the same basic information silkscreened onto them. There is the NGH number, SNK's internal cataloging system, then a bunch of metadata about the mask ROM itself, such as when it was made. Toshiba made Mask ROMs tow this line, but sometimes SNK used cheaper mask ROMs that only have the NGH number printed on them. Extra codes like P1, C1 and the like are placed after the NGH number to denote what kind of data is stored on the Mask ROM and where it goes on the boards.