Joe Martins is a 3/4" side micrometer side mount, CMHJohns is 3/4" a manually adjusted side mount, Tripleduce has a 5/8" center mount and I have a 5/8" micrometer side mount on my 22HP. I think 5/8" scopes were discontinued around WWI.
This link has some information on the differences -
/malcolm-scoped-1899s
