Hi John,
There are two fuselage lengths for the Bo105 the standard 105 is the shorter fuselage while the 105CBS is lengthened by 12" so it can accommodate a larger stretcher unit.
The easy way to tell them apart is that the CBS has an extra window behind the rear cabin doors. There were other minor changes, but that is the big tell.
I don't know what version the Airfix model is.
I hope this helps.
Grubby