The QC gear box dates it the early 1920’s, maybe a bit earlier. I think that style gear box stuck around until the 1930’s on some machines. Look for an inspection plate on the end of thegesr cover mounted on the headstock - if the plate is still attached, it will tell you the swing, length and which catalog number that SB originally listed it for sale. Might be a SN on the tail end somewhere, but the catalog number will be the defining criteria for determining age (SB had several different numbering schemes over the years, but only one sequential catalog number series).
Don’t let anyone tell you this is a boat anchor just because of its age. My SB 9 x 48 was made in 1924 and it still has original scrape marks along the bed and crosss slide. It’s still as tight and solid as the day it was sold. These old barn finds are still around- and some are in excellent condition.
Yet Many are clapped out, certainly. So when you go look, do a T/I for wear on the bed and under the tailstock. And try to pin down what the owner knows about it’s history. If it’s been in the family, sitting under a tarp in the garage for 80 years, might be a very nice machine!
Glenn