Hi Ronald,

U-boat guns were not marked with the number of the U-boat. Only things that were unique to a particular U-boat were marked that way and there were very few items like that on a U-boat.

I don't know what the 77 meant for sure but it is common for German military weapons to have a some of their parts marked with the last two digits of its serial number though I don't know if this applies to deck guns and I don't know where the serial number is on a deck gun either. Looking for "matching serial numbers" is how gun collectors determine if a particular gun has all of its original parts.

Regards,

Ken Dunn