Georg,
Looks like a very nice one! The finishes appear correct to meI agree that the barrel is a replacement I suspect that the stock was replaced as well since it only has a single proof mark on the grip.
The serial number on the scope is the electro-penciled number on on the scope data plate (#32XXX). The other numbers you have observed in the 7,XXX,XXX range are drawing numbers. The late model M73B1 has three of them, one for the scope, one for the data plate and one for the tube. Drawing numbers served as the kernel for replacement part numbers in the ordnance supply system.

Again nice find and a great price.

Regsrds,
Jim