Hi Chris...it's hard to explain all of the things to look for in a brief reply. I've specialized in Mobil for over 10 years now. I would suggest using the Old Gas search box and going from there as lots of the details in looking for original Mobilgas plates has been discussed before. In a nutshell, the IR52 date code is in the correct place for a wide blue border pump plate. There are literally dozens of different Mobil plates out there, so there is much to learn.
I do commend you on doing research to find an original plate. It's upsetting watching the market on these signs suffer from all of the junk that's out there, and not just Mobil plates/signs. On one hand I say buyers deserve it for not doing the research, but the other part of me keeps posting the repros to the "unmarked reproduction" section here on Old Gas to try and help folks from making mistakes as repros are and will keep getting harder to spot. It's getting difficult to keep up with all of the new to the market Mobil repros and fantasy signs appearing so frequently, but I keep posting them!
Always looking for Mobil items I don't already have!