I was going through my box of engines today and found an FP 40 that I bought off a guy at Hunterville a few years ago. It has been converted to CL with a venturi and an OS needle. I don't know if any other work was done to it but it has the letters SD stamped on the bottom of 1 lug and the #3 stamped on the other. Does anybody know what it means? Did they come from OS like that or did someone do some rework on it? Thanks