As a follow up to the story.

I built myself a chamber gauge, .774" dia, and marked out in 1/4 inch increments more or less matching what lengths of shells are available.
I turned the gauge from some scrap plexiglass that I had on hand.

When dropped into the chambers, the 2 3/4" line sits proud of the chamber by about 1/16", or about 2mm. Pretty much what I had measured before.

I still have not heard back from the U of G Archivist.

How long is usual for the turn around time from contact?

Cheers
Trev