It probably has to do with revisions to part numbers. I just went through this whole shebang when extending a remote IPE. The A0773054 is a 1-4 Superloop, Multi-Mode Fiber, Non-Redundant unit. I'm not sure what the other part number is. There are a lot of options for this setup, if you want to send me a PM, I can walk you through some of it, if you are not familiar with the Multi-IPE fiber remote. It's a great product, but there are some gotcha's
when doing it yourself.
Good Luck!
Wherewolf