As Nick says, they are DIN 6343 collets, size 125E (or F17).
The parallel portion that locates the collet at the back is 17.5mm dia.
The major diameter at the fat end of the taper is 22mm.
The taper half-angle is 15 degrees.
Overall length is 51mm
Looks like Santool in Germany still sell them in 0.5mm increments from 1-14mm at about Euro 20 each;
Santool online shop – Collet Type 125E, F17, DIN6343
Beaten to it by John..
Edited By DiogenesII on 17/02/2022 18:36:10