It looks to me as if it could belong to to a Chaenomelles, the japanese quince and not sure but it may be used in making jelly.

It looks like a quince to me too, Roasterman. As Julien says, you can make jelly with them (like jam but without any pips or fruit fibre in) but you'd need a lot of them to make this worth while. I don't think you could eat them raw, like an apple. In one of my garden books it suggests that they are edible when cooked. You need to let them ripen to a yellowy colour first though. Annie

Julian thanks for that. I wasn't sure if I was going to make 'poison' jelly or the old quince jelly that the Romans used to make

They do not have a lot of flesh on them but they are very strongly flavoured. A nice thing to do with them if you only have a few and not enough for jelly is to cut off the useable flesh from a couple (that's all you need) and add them to apples in a pie or crumble. Yummy!

If you were going to make jelly you'd need to cut them up and boil them soft, and then strain the juice through a jelly bag. Then add a pound of sugar to a pound of fruit as usual. Make sure all the sugar is dissolved before it gets to boiling point.

Cydonia Quinces are the ones the Romans used of course. I don't think the Roman empire had any influence on Japan! Lol.

As said above though, Chaenomeles Quince can be used in cooking too.