I thought this was a pretty unusual model of JP mower and I guess the silence confirms this. I wonder if it was a prototype, and what is the V pulley doing on the outside of the roller as the roller drive is already driven internally from the engine?

Thanks for that observation, the extension appears to be necessary to enable the user to lift the idler pulley in and out of gear effectively and the red catch higher up holds the lever out.
There are no controls on the handlebars you would expect on most motormowers and the JAP engine appears to be a fixed speed.
However the engine is painted the same colour as the mower and there are no signs of any other colour underneath. If it is a conversion of a push mower a lot of thought must have gone into it.

Eventually found some numbers, The JAP engine has what appears to be 2AV 85976 pressed into the cowling, although the V could be a U. According to a JAP dating list I found this makes it 1956 or 1943. I’m not sure when they began making 2A engines but since this one had a recoil starter I am guessing it is probably 1956.
Taking a closer look at all the JP mower pictures I can find I now think this could be an early style Maxees push mower which someone has put an engine on. All I have found so far on it are three letters G V E inscriber on the sideplate near the back roller.