The markings are pretty much self explanatory but I have not seen them before. The J frame SB magnas with blue washers are marked on the left panel with a J and the right panel is marked with SQ and what looks like a J in front of that. The mark in front of the SQ is difficult to make out. No serial number and no blue ink. I'm thinking factory replacement stocks but I just don't know. Does anyone have information on these?

They are factory replacement stocks as you supposed, just that simple.