Hi Trev,
One of the Webley Juniors pictured on the factory display board during the 1970s appears to have had the same cylinder hole. Not sure why, as the pistol was fixed to the board via external clamps from what I can see. A black & white photo of the board appears in a 1970s issue of Guns Review as well as pg 155 of the 3rd edition of The Airgun Book by John Walter.
Kind regards,
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.