[Ex Shack]

Hi Robert
My apologies if this is telling you what you already know
The a/c ,as an individual item, would have a unique serial painted on the side of the fuselage towards the rear and aft of the RAF Roundel,this would be put on in the factory and taken from an allocated batch issued when a number of a/c were ordered.It would,for most Blenheims, be a single letter followed by a 4 figure number say V5757. There would be a Sqn Code,2 letters on one side of the Roundel and a single letter on the other side.The serial would be fairly small but the Sqn ID would be at least as large as the Roundel and the 2 letters would show the Squadron ,the single letter being for the a/c .
Inside the a/c most serials would be for individual items of equipment and would be on small plates attached to the equipment or stamped into a solid metal part like an engine block although even there they often put a small plate. There should be an inventory,which would record the equipment including the changes when a piece was replaced.
Look for Bristol Blenheim on Google Images and you should find many instances where the external markings are identifiable and this will show you where to start looking on a wrecked example, The markings were not totally rigid but did follow a pattern.
Regards
Dick