Thanks OldSalt1841; that is indeed the number 791, which is the given weight of the barrel (it matches the wt. recorded on the Registry).
The large threaded vent hole was meant to accept an insert called a bouche, which was usually made of copper, and was easily replaced when 'burned out'.
I wasn't able to get in contact with the pres. of Kappa Alpha Fri., so I'll try again on Monday.