Diameter of stem primarily, and maybe its legth. The idea was to see which toolrests are interchangable between models and makes if they come up for sale (cheaply ), or if changing lathes whether they are compatible old to new,
3/4 in stem (as above)
50 mm stem length to shoulder, (60 mm would still clear the bed in lowest position)
70 mm from shoulder to working edge of the rest (on the standard 200 long rest as supplied) making total height 120 mm.