The placement of the thermostat housing is always a good one to tell whether it's a J or an A.

You never know for some bizarre reason it may have had an A15, but not likely. People have sold J15's in the past for a few hundred saying it were an A15 (ebay) so some poor folk have been fooled before. Almost happened to me once, until I received the photo's of the engine. Good luck, but seeing as it's dropped a valve or burnt a valve I reckon steer clear.

Here is a J-series: