miles off lol
ahh well no worries
This appears to quite literally be the case.
The D.521 is just the D.520 with a different paint job and un-upgraded. (Despite being a premium...er... How does that work?).
Some investigation, and discussion with Mr.Matt_FSR, has found the following:
-The experience of grinding kills out for this, while good for our Lion count and a good demonstration of skill (Both of us, and I'm guessing EssexJay also, were always in the top end of the team, usually 1st, in every battle. 1337.), was unsatisfying given the D.521 does not do anything the D.520 doesn't do. Hence much effort for very little.
-The few sources of information I have found state that the Merlin III engine used in the D.521 does not allow for the fitment of a through-the-nose cannon. The D.521 that existed (there was only ever one, modified from a stock D.520) was not armed, being purely experimental. But the plans were to fit it out with the same armament as the Spit 2b! 2x20mm and 2x7.7mm, wing mounted. But the D.521 in-game has the through-the-nose 20mm and 4x7.7mm wing mounted guns. Exactly the same as the D.520. Because lazy Gaijin.
-This one is just me, but it's god-damned ugly. Just the paint job. It's like a very cute lass, and they've dressed her in leopard print. Blurgh.
Nice wings tho. Dem invasion stripes tho.