The FV & FVP are about 56cc, the LS is 64cc
It seems to be that the F, stood for Forty.
FV, Forty Villiers, fitted with a Villiers carb, small gearbox, commonly known as a forty minus.
FVP, Forty Villiers, with a bigger gearbox or plus box, commonly known as a forty plus.
The LS, may mean Long Stroke, compared the early model fortys and so it was called a Super model forty, not a little forty.
Most common attributes to the little forty motors, not the LS, is that the tiller arm fitted to the centre of the crank case, the bronze transom bracket was plated as was the fuel pipe, the drive shaft tube is clamped to the base of the crankcase, the drive tube is also screwed into the gearbox, not clamped, you do get variations as this series was developed and improved.