MPN11,
So were we originally, until I found Wayne Brown's photo caption, which states that the 0.50 and its matching canopy section were replacements (what happened to the original 2x 0.303s ? - I suppose that as an instructional airframe they were not necessary. But they could have been kept (with the breech blocks removed) as part of the whole aircraft).
Yes, the AoI is the key. Trouble is, all Vengeances look alike, it is impossible to see the AoI on photographs and even with the naked eye I would think you would need a rigger to make a certain decision. The gun mountings in the wings should be another recognition feature, and of course if the guns were there, too, that'd be game, set and match ! But Megan's pic shows an empty space where the outer gun would have been.
Time will tell,
Danny.