Here is some info on Darne grades for you to look at:
https://doublegunshop.com/darne.htm
What you have is a R15 Grade which is one of there middle grades. Yours is a later built Darne with full 2 3/4" chamber indicated by the 70mm on the barrel flats. I would venture a guess it was built in the late 60's into the 70's. Many of these were imported by Stoeger at the time, but they were usually marked Stoeger on the barrels. They are a very well built gun and you have a beautiful English Stock on your Darne.