Here's what Benjamin Seaton listed on his inventory of tools as a sash saw. A common designation for what we would simply term a tenon saw. Trust me, the one he termed a tenon saw was much larger...
14" in blade length, about 2 3/4" of usable depth at the toe and rising towards the heel of the saw. 13 ppi rip. Brass is about 7/8" by 1/4" and in this case is slotted vs. folded as would be traditional. Brass blots and split nuts. Saw plate is .025" thick. Handle is German Beech.
Sash saws, much like our common tenon saw, came in varying sizes and were actually used on a wide variety of work, not just window sashs. As Benjamin Seaton was a trained joiner [furniture maker] it may well have been purchased as an intermediate tenon saw, placing it between the carcass [rip] saw having an 11" blade length and the large tenon saw having a 19" blade length.
Take care, Mike
Well, back to work...