Well unless you talk to someone who worked at HK at the time in the stamping department I doubt you will ever get the absolute truth. (And I would be highly skeptical of anyone who posts anything here as absolute truth! :D )
I know that some HK91's are marked 7.62 and some are .308. You may have just gotten one of each and 911/91 thing has nothing to do with it.
Out of curiosity, what will the definitive answer tell you? Since they both can take either .308 or 7.62 there is no real difference what it's marked as. Much like the SL8 is marked .223 and the G36 is marked 5.56, the receivers are (virtually) the same and both can eat either ammo.
Also, the title of your post seems to indicate you think that the chamber is actually different. It's not, they have the same chamber, they are just marked differently. HTH.