IBM 5161 - Cable
A description of the cable is in IBM's technical document for the 5161 at here.
Pages 1 and 15 (PDF pages 5 and 19).
A non-IBM replacement cable can be sourced from here and here.
Whilst it may be tempting to purchase a very long cable, that can cause problems (more signal delay, more signal degredation).
Probably best to limit yourself to what IBM used: 1 m (3.28 ft)