Here is a link to Griffin and Howe's instruction page for installing a side mount.
http://www.griffinhowe.com/mountsandpads.cfm
you have to scroll down within the box to find the instructions. I get the impression that
the base plates were shipped without holes and so a gunsmith might have gone with any
pattern of holes and pins that he liked. I don't know if Jaeger sidemounts were shipped like this as well.