This is from one of Monarch's chuck manuals...
THREE SIZES OF cAMi lOClt SPINDLES
Monarch has standardized on the cAM@lOClt spind~e, three sizes of
which cover the entire range of Monarch lathes as follows:
The Standard D-1-3" nose is used on the Monarch 10" Toolmaker's
lathe, l O" Manufacturing lathe and 1 O" Manufacturing lathe with turret.
On all lathes of 12", 14", 16" and 18" sizes the D-1-6" nose is used.
On 20", 2211
, 25" , 3011 and 36" sizes the D-1-811 nose is used.
All chuck manufacturers make chucks with steel bodies to fit directly
to the cA#llOClt spindle nose without the use of adapter plates. The
cam-lock studs and lock screws needed for each chuck are supplied
with the chuck by the maker.
THE MONARCH MACHINE TOOL COMPANY