Thanks for all the replies.
It's fairly obvious to me now that I'm not going to establish the exact cause - I'll just leave my readers a footnote saying it could have been any small asymmetrical feature on the aircraft, intended or otherwise. Does that sound OK?
The information comes from Dutch service pilots and Finnish military test pilots. The latter were pretty thorough and their information that both the Mercuy and Twin Wasp versions stalled the left wing first should be reliable.
However, my source says 'flight reports tell that both stalled first to the left and immediately afterwards to the right.' This is not exactly my layman's view of what happens when an aircraft stalls into a spin, but maybe that is just me, or a translation problem?