Hi, it is a coarse spline and if I remember correctly, uses a TR 2 - 6 8 1/2 inch clutch plate, also you need to get prop shaft shortened to accommodate the longer gearbox with O/drive.
Please note this is not my g/box, I am only posting on the forum for a friend as I thought it might be be of use to Commer fans. Please phone Chris for more details. Thanks, Trev.