Your site is absolutely amazing. I have never seen a clearer explanation of SB serial numbers.  I have three old lathes.
 
Lathe #1 - 9" Mod. "C" with a forty-eight  inch bed. Horizontal bench drive with the original motor, single speed.  Came with a basic tool post some dead centers, a few gears and a 3 jaw ( and 4 J. - no back plate) chuck. Just enough stuff to get going..It has an oval tag reading "METCAN 651072"  attached to the bed with drive screws. S/N 58931  which would date it back to 1935.
 
Presently, there is a discussion on over-sized spindle threads on Milicron's board. Well, mine also has one. It measures 1.564 " OD, so the back plate on the 3J chuck is a one of a kind, and any SB or purchased accessories won't fit. My guess is that the spindle is original, no indication that it could be an "after market" piece.
 
My solution was to change the head stock completely. Now the 3J is loose on the new spindle. The solution to that was to wrap a single layer of aluminum Coke can around the threads to act as a bushing, and it runs true again. Not too scientific, but it did work.
 
Have owned this lathe for about 20 years and only use it occasionally for making or altering small things for my old trucks. No big or fancy projects to date.
 
Lathe #2  9" Mod "A"  S/N 27619NAR9  which would be about 1952. It has a H. bench drive, single speed original motor, with reversing switch. No tool post, drive belt or tooling. It has one gear in the gearbox that is missing three teeth, and am in the process of fixing it. The bed is shorter, about 36" or so.
 
Lathe #3   9" X 36"  same as above except a Mod. "B". No tooling or access. but does have the original motor/H. drive with rev. switch as above.  I cannot access the S/N at this time as it is put away in the garage in a corner.
 
I bought lathe #1 from a friend who found something better. And I got #2 and #3  from a part time dealer.  They came from a school shop, guess that explains the stripped gear. Originally I was at the dealer's place  looking at a Rockwell wood lathe and when I was about to leave he asked if I was interested in a couple of metal lathes he wanted to get rid of. Didn't take long to get them off that huge table they were mounted to, when he suggested $100 for both. I let him have that table which weighed about 400#.  That was more than 10 years ago.