250-100 designation is for BXA sized toolposts.
clipped from post elsewhere :-
The "A", "B", etc. type designation is Aloris sizing. Yuasa, Phase II, etc. use 100, 200, etc.
AXA = piston type = size 100 (2 1/2" wide, 14mm dia t-nut) = 1/2" tool bit, 3/4" boring bar = model 250-100 .
AX = wedge type version in size 100 = model 250-111
BXA = piston type = size 200 (3" wide, 16mm t-nut) = 5/8" tool bit, 1" boring bar = model 250-200
BX = wedge version of size 200 = 250-222
IHTH
regards