Did you really mean .065" at the forward end of the chamber?!?
The CIP "standard steel" 20g minimum would be > .075", and most smiths recommend > .090"
Are the chambers marked 70mm and now measure 2 3/4"? If someone lengthened the forcing cones, and the chambers slightly in the process, that might explain the barrel weight discrepancy.
You are correct that the weight is stamped at the time of proof. Belgian proof was after final fit and finish. I couldn't find what the Spanish proof house process was.
BTW: Spanish 1000 kg/cm2 proof = 14,223 psi + 10-14%, for a service load of about 10,500 psi
