Does anyone have any idea of the origin, or earliest use, of the term "keystone" to indicate a retail price that is determined by doubling the wholesale price?
Hi Orange, welcome
You might try Googling as follows:
keystone words origins double price
You will get a quarter of a million Ghits bt I'll bet you'll find help within the first couple of pages
Or either you could go to
wordorigins and ask there. They're good at this sort of question.