[Osprey]

OK, I found some references to the wing, it's going to pretty cut up but to summarise:

1. Supermarine specification #462 of 20 Feb 1939 outlined proposals for improvements to the production spitfire...(lists requirements of which)...to fit the aircraft with 2 20mm hispano cannon, if desired alternatives of cannon and mg.
In a meeting at Supermarine in Southampton discussed was trial installs of (a)2x20mm...(b) 4x20mm (c) 6x colt browning 50's. Joseph Smith enquired "...if there would be any objection to adapting all 3 installations in such a manner that alternatives of cannon and machine guns could be installed"

This suggests that the 'universal wing idea was born according to this spec on this day. There were many other specs too and Supermarine built prototypes for testing at Eastliegh....

4th June 1941, Supermarine tech report states weights of Spitfire III W3237 auw with A wing 6831lb, with B wing 7020lb and the new C wing (4x20mm) 7445lb (this is the first time I see 'Universal Wing' mentioned) and later it it mentions the dropped 'D' suffix for a 12 browning configuration.

Under the section on the mkV it specifically states that the Ministry had a problem with general aircraft designation (MkIII was in trials, MkV to counter the 109F), This was around Feb 1941, and was affecting the ordering spare parts in particular. Thus it was actually the Air Ministry that listed the letter designation according to gun configuration and this was applied by Supermarine.

On the subject of types there were planned 3 types for the mkIII's later abandoned but the designation system was adopted for the VIII and IX etc. The 'F' (fighter), 'HF' (high fighter) and 'LF' (low fighter bomber). There were dozens of combinations of Merlin engine and blowers and props which affected performance at different altitudes

Hope that helps.