You really need to post some pictures of this gun's barrels and the receiver flats. IF the gun is stamped XXVI it was produced in 1970. Without being able to see the proof stamps, it is very difficult to determine the proof pressure used.
You need to have the barrels measured by a professional for the chambers, choke and wall thickness the entire length of the barrels. I have an Beretta sxs sitting here that has been repaired. The repair can not be seen, until you know where to look. Apparently there was bulge just ahead of the fore end. The left tube is dangerously thin for about 1" inch there. I would not have caught this with the measurement. The gun looks great, but I will not fire it. Some how grown attached to my thumb over the years.
Pete
