There is a choice of two gearboxes. In the pictures there are some of each. The boxes came out of either Austin Sheerlines or Princesses and would have been on the backs of the 3.5 litre or 4.00 litre engines. I am only guessing that it may have been used in the Jensen 501 as it was the Princess type engine that was used in the Jensen.
If you would like to know any more please let me know.
Thanks for looking,  Graham Parsons, EGP Engines