Hi Aussietrev,
Looks like a 20" Atco. These were made between about 1946 and 1951. Engine is a 147cc Villiers two stroke. They were replaced in 1952 by the 20" lightweight which has the same frame type as the late 40's/50's 14" models.
They are quite a simple machine with quite a bit of grunt. Looks like this one is in relatively good nick. Also looks to have aluminium rear rollers - this would make it a later production. They are not the most common of Atco mowers but not the rarest either.
Price is always interesting with these mowers - the key for me is if it has spark as the coils can be a bit pricey. I would say a price around $150 would be a fair price, maybe a bit more if it has the correct catcher. I have seen one or two go for $50-$100. I have seen one or two sellers try to extract $500 from unwary punters for a similar mower in worse condition.
Cheers,
Sir Chook
