Oh - thanks for the explaination - maybe that also explains the black plastic circlip that can be slid along it - presumably to adjust the recoil in some way?

It seems strange to have this feature - but not to include it in the rifle drawing inside the handbook cover, or to explain its function. In fact there is no mention of it at all!

Do all 603 rifles have this, or was it something added to the later production - maybe this would explain why the handbook does not mention it as the handbook would not have been updated if the rifle was at the end of its production time?

If anyone has an answer for this the rifle number 317907 may help identify the production date. The dioptre is marked 4/04 but I can see nothing that looks like a date stamp on the rifle itself.