It’s clear to see from the outside that Puzzle Bobble’s label is different from the legit Metal Slug – the colours are wrong and the label is made from a different material to the others. Puzzle Bobble was commonly bootlegged due to its small size and popularity. www.mvs-scans.com is a great resource for checking labels and boards.
The boards inside are authentic SNK boards, but the high number of EPROM chips here gives away the bootleg nature of the game. Legitimate games do sometimes have them (often after repair), but usually only a couple. What’s more, the ROM chips numbered 044 show that this was originally an Art Of Fighting cartridge.