Hi Gregers,
Just re-read the article in SAM,
The Lancasters used were modified to Type 464 (Provisioning) standard and the following was mentioned about one of the Mods carried out on Gibsons aircraft:
'ED825/G was fitted with a free, ball mounted under fuselage machine gun, which is shown in the original drawings, but as far as is known, no other aircraft carried such a fitting, and it is unknown if it was still in place on ED825 at the time of Operation Chastise'.
Doesn't solve the mystery entirely, but if you weren't making Gibsons a/c in particular, it resolves the problem!