No point in getting too pedantic. The Arrow cars usually had 1725cc engines, I think, but the poverty-spec Minx variant had the old 1494cc engine.
I reckon the puzzle car had a Rapier engine fitted, or a Minx with twin-carb setup or similar tuning. To complicate matters, the Rootes cars had 1592cc engines from 1961, so the engine in this car must have been quite old when fitted.
Still can't find it, although I've found some great new sources since I started looking!