Re: 1960s Scott-McCulloch built Elgin FULL SHIFTING 3.5hp?
That little single cylinder Scott-Atwater first appeared in 1946, and was made forever. In fact, I have a '46. As I mentioned, it was made with a full gearshift for at least those 3 years. In fact, S/A was the first motors to have full gearshifts.
I don't find it strange that they may have made shift models in the 1960's. I can't say concerning the rarity though. McC also came out with a 4hp in the 1960's using their chainsaw powerhead, a different animal.
Here's the leading authority----
http://www.scottatwaterinfo.com/
EDIT: Some research indicates that the Sears / McC single cylinder motor was produced as 3.6hp from 1960 thru '63. In 1964, it was demoted to 3.5hp, the only year I've found for 3.5hp. Model number was 574.60030. So now all you have to do is find a parts book for a 574.60030 and see if it had a shift.