Congratulations. Great deal. The serial looks like B1310015 to me, which would mean that it is one of 20000 allocated September 1 1905 of model 66s at the Elizabethport factory in Elizabeth, New Jersey. I'm pretty sure that only 66s had that decal set. It would be a 66-1 as it is a back clamp.
Did you get any "fiddly bits" with it?
You can find a manual at
https://www.singer.com/support (Not affiliated with link). I think you will have to put in 66 and you will want the 66-1 manual.
Janey - Neat people never make the exciting discoveries I do.