TH44, Mark has emailed the ledger page to me but interestingly, it makes no mention of the original chambering.
I need to look closer at it as it is not too easy to read to see if I can decifer it better.
I also thought it could have been a 450/400 originally but in retrospect, the swath of wood taken from the stock to allow for the sight in the comb would have probably precluded it being a heavy recoiling rifle from the start. Also, the barrel shank on a 303 would have been the same as on the 6.5mm I would think, or at least very close to it.

Rule, the little knob is for the sight. I don't know how it works but I imagine that it may have a little disc with graduations cut out of it so as to lock the staff at the prescribed heights/yardages. Maybe I will take a look some day. It would be nice to restore the sight to original some day.
This rifle doesn't have as deep a grip as the other two with this feature so I can't imagine the sight staff has a whole lot of vertical adjustment. Maybe this is the first Gibbs with this feature and it didn't acheive enough elevation, so the later examples had a grip that curved nearly vertical at the gripcap so a longer staff could be fit. Who knows...