It would be a useful though rather esoteric project for all the other post offices, perhaps as part of a scanned and recorded census of covers of the period.
I have some, and also a stamp that bears a very likeness to Joy's
P&O cancel, the "S" is particularly close.
The benefit of keeping many duplicates with Postal markings
Results of the small study
Style of marking
1 2 parallel lines (largest group)
2. single horizontal line (next largest)
3. one or more parallel lines on an angle tying the stamp to the cover
4."stamped" hand written
5. Name or Initials without lines
Joy's cross hatch matches item marking #278
Perf12 Cover "STAMP" and intials EK match #234