Raimey,
thanks for that.
I would be very grateful if you could find any information on the serial number. As according to answers from Germany the number - judging by a photograph provided - seems to be original (without an 'added' figure), there must have been one of the makers in Suhl who used that particular range of serial numbers in 1939.

Do you have any info on what the interlaced J&Y (or 'double duck')might stand for?
I have made a comparison in PhotoDraw of your picture and the marks on my gun and attach it here.
I absolutely think you are right and that the marks are identical. Please correct me if I'm wrong

Having spent the last two years trawling the forums on my quest
and taking the information found, mainly provided by this forum, so far the possible makers of my Otto Bock BBF seem to be Schmidt&Habermann/Oscar Merkel or possibly Bernhard Merkel.
The main problem still is the question of the high serial number.
Comparing the pictures I have seen on this and other forums on german double guns, the action of my Otto Bock seems very similar to the above mentioned makers.
Perhaps someone can confirm my observation or confute it?
Here a picture again:

Best regards from England
Günter
NRA LIFE 1974