I can tell you Starrett & Lufkin Calipers are the most popular. The Starrett differences are; Toolmaker Spring Type No.274,275,277 2-6” Caliper OD/ID/Divider = round leg / solid nut Fay Spring Type No.74,75,77 3-6” Caliper OD/ID/Divider = flat leg / both solid nut & spring nut Yankee Spring Type No.73,79,83 2-1/2 - 12” Caliper OD/ID/Divider = flat leg / both solid nut & spring nut - with threaded nut attached to the legs Firm Joint No.26,27 3-36” Caliper OD/ID/Divider = flat leg / both solid nut & spring nut - with threaded nut attached to the legs Lock Joint No.38,39 4-24” Caliper OD/ID = flat leg / Fine adjustment screw Lock Joint Transfer (for Layout) No.36,37 4-24” Caliper OD/ID = flat leg / Fine adjustment screw (I sell them all day @