I had a group of 5 turn-of-the-last-century Disston backsaws - an 8”, 2 10”, a
12” and a 14” - that I sent to Tom Law perhaps 8-9 years ago for refurbishing
ang shrpening.  We talked about ppi, rip, cross cut, etc, and when I mentioned I
would like the 8” at 15 ppi rip for dovetail work, he said he couldn’t do more
than 13 ppi any more because of his vision.  It came back 15 ppi anyway.

So I think it is just a technical issue.  I have a 1970’s Tyzak 8” with 20 ppi.

Ed Minch