I think he got confused by the stubs outboard of the 20mm's.
The stub would represent the area where the other 2 20mms could have been fitted. As it is an early Mk IX based on a Mk V airframe the layout is correct.
Also the small blister on the upper wing is correct.
Skuzzy - cc, just thought I'd point it out in case it was an error.
Maybe model the V with a large blister if the occasion ever arose to give us a 4x20mm spit V (hint)
NICE JOB by the way, looks great!!!