Ross Mk III, hole in underside of the receiver reinforce

I've just acquired a Ross rifle and upon stripping it down to check it over and give it a clean I've noticed this strange counter-bored hole underneath the receiver reinforce.

I've found a Ross handbook online and I think it's the bolt unlocking pin / retaining screw location (shown as "T" on the main drawing). It doesn't look like it's been there for quite some while and I don't think it'll affect the use of the rifle, however I'm curious as to what it is!

Can anyone shed any light on it, or whether anyone else has one in a similar condition?


Thanks for looking, Jon Attachment 139207Attachment 139208Attachment 139209Attachment 139210Attachment 139211

Yup that’s what’s missing. It also helps secure the barrel in place.