1 x 4225C. This is the rare and easily lost " floating"  column part that inverts inside the Later steering nut to lock the spline against the upper column.

For some reason Jaguar misleadingly named this in the XK150 Parts Book as a 'Split Collet'  when it is actually NOT a split 2-piece assembly like the steering wheel collets behind the boss, it is a complete one-piece, 8-finger collar that sits the opposite way around to the earlier column collet fingers, inserted into the Later design  Hand Locking Nut ( SEE : 4225 ) as shown in the last picture! Original Ref. C13638.