Hi Alex. I haven't been able to find anything definite on the particular aircraft you posted the pic of. But you are right, only two Squadrons used the Whirlwind, 263 Squadron and 137 squadron, and the squadron codes for 263 were HE. (Those for 237 were SF).
The Whirlwind displayed the codes in a (slightly) unusual way, with the individual letter forward of the RAF roundel on the port side, the opposite to standard practice. I've only been able to find the following individual codes and serial numbers for 263 Squadron so far;
H-HE serial No. P7011
L-HE serial No. P7062
V-HE serial No. P6969
Pictures of 'L' and 'V' do not show the shark's mouth marking, 'H' is unknown.
If I find anything further, I'll let you know.
Terry.