G'day Max, Paul, WCE Max, many thanks for the clarification.
4BM recoil, 4bIM for impulse. That is how Turner advertised them on release. Yes, Max, I feel the 'I' did designate Impulse. I think the 'B' reflects the new non-Carter chassis? [In many ways, Turners first chassis design]
I think changes to the engine - like the move from Bing to Zenith carbies would be reflected in Kirby model designation, not Turner's.
