Look at the details of the chuck on your invoice, or on the RG Tools website. It should mention the fitting, maybe JT–, or B–, if it is not already marked on the chuck. The tailstock size is definitely MT2 (Morse Taper 2).
If it is RDG Tools you bought the chuck from, then the three most likely chucks, judging from your photo are as follows:
1-10MM CHUCK = B12 fitting. Order Drill arbor MT2 X B12 with tang
1-13MM CHUCK= B16 fitting. Order Drill arbor MT2 X B16 with tang
0.5-16MM CHUCK= B18 fitting. Order Drill arbor MT2 X B18 with tang
Edited By old mart on 09/12/2019 20:39:57
Edited By old mart on 09/12/2019 20:41:20