Here are some pictures of three covers that are in my Sten parts bin.
The top one is marked “S68”, the center one has a C with a broad arrow inside and the bottom one is “S106.”
The S68 (Lines Bros Ltd) is welded at the back. The outside was linished, making the join hard to see. It looks smoother than the LB marked cover.
I believe the "C broad arrow" is the CanadianGovernment property mark. It is welded at the back.
The S106 (Spear & Son) is like the ones you have, stamped only.
All three are slightly different. The S106 is closest in the picture. It has a larger radius at the back than the others.