This question rather puzzles me. Haven't you got one in your shed to measure? We had someone on the Drummond forum a few days ago asking the same thing when he had just admitted having the lathe but seemed that taking the existing chuck off to measure it had not occurred.
Then last year we had a query about the drawbar thread in an INT40 taper so I and a couple of others went to our sheds to measure and concluded there was more than one standard. Fine if the questioner had been wondering what he might need in advance of purchasing a machine with a new to him style. But the blighter had one right in front of him.
Equally not impressed by Americans who measure a bolt as 1/2 inch x 12 TPI and can't google a bit to discover that the whole world is not wedded to UNC 13TPI.