Hi Mike
AFAIK, the 1924 pattern had a redesigned hull and a new turret with a cupola for the commander. I think the wheelbase was longer too.
Here you can see the 1920 pattern on the left and the 1924 pattern in the middle. The one on the right is the modified 1924 pattern with the open-topped Frazer Nash turret.
More bits of information
hereHTH
Frenchy