Hi Lewis .....Ref the FE4
...It was originally designed for RAF 3A engines and during this period , new modifications were put underway in order to install RAF5 motors , the engines being mounted on steel tube bearers .
Later conversion drawings were put in hand to replace these with the later Rolls Royce 275 hp motors .
There are a number of miscellanious original drawings still in existance for this machine ,; about 160 appx I believe , (Avia 14/25/1/1-90 and avia 14/25/2/1-58 plus others with dual comonality ).obtainable from the National Archives.) though not enough to build from I strongly suspect ...Some parts such as components within the controls were shared with RE7 etc .The machine had some armour plating and was designed from the outset to have Olio landing gear .....It wasn't a world beater !
And Now back " On Topic " to the FE2b..

.( Just got to now resize some more photos for same )
Regards John