a,b,c,e only referred to wing types and had nothing to do with low/high backs.
Same confusion arises over the often misquoted IXC.
IX's were originally all c winged, so were only referred to as FIX, HFIX and LFX.
a,b,c were only used up to and including the V's. After that there was only ever an e suffix.
There were also a small number of low back LF.IXe's.
The 2 prefix on the Merlin applied to all Merlins manufactured outside the UK and was used long before the Merlin 266.
The tooling issue was resolved by designating Spit IX's with a Merlin 266 as Spit XVI's. Not by the 2 prefix on the engine.