Random thing I noticed while flipping through the SHP list:
{9423, 2159}, // 2223 Mk V (Male)
{9424, 2159}, // 2224 Mk V (Female)
{9425, 2159}, // 2225 Mk V (Hybrid)
{5477, -1}, // 3077 A7V
{5478, -1}, // 3078 Saint-Chamond
{5479, -1}, // 3079 Mark A Whippet
It's now possible to basically, if you're judiciously careful, have SPWW1 battles with stock SPWW2 within constraints of the engine (i.e. earliest battles in 1930) with no extra modding (SHP files) needed; since you can then use other icons already in game for what limited air support existed in 1917-1918, i.e.
Westland Wapiti (UK Slot 798 - Icon 7967) for Royal Flying Corps Support
Curtiss P-6 (USA Slot 360 -- Icon 856) for United States Army Air Service
Potez 39 (France Slot 565 -- Icon 8261) for Armee d'lair (France)
He 51 (Germany Slot 340 -- Icon 545) for Imperial German Flying Corps
Plus, many of the guns of WW1 were recycled into WW2 -- and some exist in other sections:
BLUE OOB has Unit 328 -- 105mm mle 1913, etc