Thanks Chris. It could be that the older bike is a Heinz 57 and the latter, when he could afford something better, the real deal. I'd be really interested if anyone can shed further light on the older one.
Thanks Eddie. What you say about later bikes having re-positioned kick-stand and the pancake dynamo makes a lot of sense. From what little I've read, Douglas were getting into increasing business difficulties around this time and started putting out a lot of models with minor changes but very short runs. This wonderful website lists the A31 but has no pictures. I Googled and found these two links:http://www.kynoch-douglas-parts.com/html/wanted/main/douglas%20parts%20wanted.htmhttp://classic-motorbikes.net/gallery~douglas-classic-motorcycles
They look very close and both mention 350cc so is that likely the capacity of my dad's? Would this bike have been purely a road bike or did it have amateur dirt/race pretensions? I should explain that my dad's pre-war life is a bit of a mystery to me and my sister, but I believe he did have a strong love of motorbikes and racing and might have been competitive. During the war he was a Spitfire mechanic, and later turned is petrol head to sports cars.
With very small build numbers for these bikes it does seem like more than a coincidence that the RV2289 is indeed a misprint. That's exciting. If I get in contact with LDMCC are they likely to be able to tell me anymore than you have?
Chris, Eddie, thank you for your help. I'm very glad I asked and am getting more and more interested in these machines.