Good info, but a bit out of the ordinary. I'm not doubting the sources, especially as I haven't seen the photos, but if possible, check further.
The reason being, both the cannon mount, and the 'stub', were actually fairly substantial structures, being heavy 'tubes' mounted through the wing leading edge, back to, and through the main spar, into which the weapons could be mounted. Bear in mind that the 'C' wing was designed to have two cannons per side, as originally fitted to the MkVc, but the outer mount was left 'empty' on the MkIXc. These 'tubes' ended roughly six inches or so in front of the wing leading edge, where they were then threaded or flanged to accept either the tapered 'sleeve' for a cannon, or, when not used, the domed blanking cap.
It is of course possible that the unit made a 'local', field modification, removing the outboard 'tubes', presumably to save weight and reduce perceived drag, to gain an extra mph or two, but it's also possible that this might be a result of faulty photo interpretation.