Yes, the receivers of rifles built entirely at Long Branch are marked that way -
However, the ones assembled using up Savage-made receivers are usually marked on the wrist socket with the stylized L/B logo (which was used mainly on bayonets, magazines and such) to show where the rifle was finally assembled and serial-numbered.
Here, on the other hand, is the "S"-marked wrist socket of a rifle fully completed at the Savage plant in Chicopee Falls, Mass. -
Oddly enough, if you Google the subject of Savage-made No. 4 L-E rifles, you'll see many people expressing the view that they are generally more poorly finished and fitted, and also tend to be less accurate, than any of the other No. 4 rifles made elsewhere .... whereas Long Branch rifles are often touted as the
best in over-all fit, finish and accuracy!
However, I don't recall seeing any sort of reconciliation of these opposing views in discussions of the "hybrid" Savage/Long Branch rifles!