I found some museum archive records in Austria and Germany and 1937 seems to be the date in question. I'd been after this particular image for about a decade and couldn't believe my luck when one finally turned up deep inside Eastern Europe. I was initially unsure about buying sight unseen - especially as it was from a private individual rather than an auction house - but it is absolutely 100% pukka. Pleased as punch!
Lovely Philips poster by the way - very ethereal.