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

if its stamped like this, that's a factory marking

  It is. Thanks for the reply Dane.