Just a minor clarification from earlier in the thread: IRRC the tube diameter of the PPCo. scopes was not changed, rather some of their mounts were made with enlarged rings to mount 600 or so Aldis No.2 or 3 scopes that were "unattached". Their later Patt.18 scopes were of course smaller in diameter, but never mounted offset as the typical PPCo. and the 600 odd Aldis were.