Put in a Google Search for:
"Si" marking on Mauser rifle barrel
(use the quotes)
You'll come up with quite a lot of discussions about the marking as found on the Mauser rifle bbls and some on the recv'rs as well.
I personally have no idea what it stands for. But the general consensus of the internet crowd seems to be that it's the ID code of the steel source Company for the raw rifle bbl blank.
The numbers possibly a steel lot numbers or other identifying numbers. Date possibly?.
Some say (guessed) the Si meant Siemens.
Another source lists it as 'Unkn' and seen on examples produced by JP Sauer.
Here's one discussion about the bbl blank supplier codes and mentions 'Si' a couple times:https://forums.gunboards.com/archive/index.php/t-50939.html
If the Waffenampt # on the markings is WAa135 then those are of Oberndorf/Mauser inspection . They used that assigned# in 1935 and again from '41-'45.