Hi Jack,
I too wondered that. I suspect that as the petrol Standard and the electric Standard were quite similar that it may have been a bit cheaper than the Centenary.
What is interesting is that the same ledger shows a similar mower in 1936 being fitted with a Villiers engine with a prefix of WL and a Centenary from the same year being fitted with an engine with the same prefix. Up until after the World War 2 Villiers used letter codes to define variants of engines, however may of these are not known. I would assume that the WL and WLA prefixes are variants of the same base engine. The WLA engine appears to have similarities with the Mk XIC, particularly the head design and decompression valve. The big difference with the WLA seems to have a different flywheel and a cowl (looking at photos of the Centenary it has the "normal" flywheel and alloy cover). The flywheel fan and cowl on the engine showed on the Standard are quite different from the engines used on Atco and other mowers from the 1930's onwards - it is questionable as to whether these would have been produced by Villiers. In terms of engine availability I would suspect that the engine may have been prewar stock.
Finds like these are certainly interesting as they tend to create more questions than they answer.
Cheers,
Sir Chook
