How do I ID the .ipsw my iPhone backup was made with?

I backed up my iphone 4 before taking it in for repair. They offered to replace the phone instead for a cost and i agreed. Upon plugging my phone into iTunes and setting it up, it said i needed to set it up *then* restore from a backup. The current .ispw is 5.1.1 and when i right click and select 'restore from backup' my backup from november 14th is not in the list of backups available. I see my wife's phone's backup, and my iPod's, but not the one I made through itunes with my old iphone 4 on 11/14.

I've deduced through looking through other posts (and i could be way off) that itunes will only see the backups that match the current software on the phone. Can anyone confirm this? So this is my conundrum. I have iBackup Viewer and can select and see the backup I want, but I can't see what .ispw it's with. Any suggestions?

Thanks.