Hi kirky.
I think 'partition' is a bit misleading - it's just the outline of the simple shape that has to be coloured in.
(Unless I've missed some oblique meaning.)
The shape is a sign that all qualified drivers will have displayed on the vehicle they are driving at some time.
None of the letters within the shape are M to Z.
27a, 39a and 23d are the only non-partition-crossing words that appear within the shape.
Good luck!