For 66's made in 1914, there were only two options, both originally back clamp machines
66-1 for Treadle
and
66-3 for hand crank
The other thing - a lot of Red Eyes got modified over the years - updated to electric, swapped the back clamp for a side clamp bar, updated bobbin winder if they put on a different flywheel. Often they would get an updated tension assembly if that went bad. Add a specific type light, and other than the decals, you've effectivly changed the model from a 66-1 to a 66-xx.
One of my Red Eyes that had been converted to Electric has a lot of non-Signer updates taht I replaced with Singer ones - probably got updated around 1940 or so.
I own a 66-1 from 1909,
66-1 from 1922
66-5 from 1923 (Same as the 1922 model but side clamp and hand crank)
66-6 from 1928 (about the same as 1923 but different decal pattern and has a solid flywheel and motor)
66-18 from 1945 (this one has a back tack and newer bobbin winder)
66-16 from 1946 (same as 1945 but with a different light assembly)
if you go to this page and scroll down to the Model 66 section, you can see notes on the different extensions. BTW - "rear mount" is the same as what I call "back clamp" presser feet
http://www.ismacs.net/singer_sewing_...sses-1-99.html