This is all speculative without the pictures but,

1. The owner controlled what was done to the gun as witness the articulated front trigger. This means that he chose to have the gun barrels done in a manner that fitted his use of the gun and probably to keep the pair reasonably matched. That would explain the 2.5 inch chambers although so would the guns being used in the UK.

2. You have been told the gun was rebarreled in 1964 and I have no reason to doubt the owner of the #1 gun who has apparently done his research.

3. Value is still up in the air as we don't know what might have been done to alter the gun since 1964. One big question is whether or not the gun is in proof--that is, do the current bore measurements and condition match the marks from 1964?

4. It is highly unlikely that the current Atkin, Grant & Lang establishment were involved but that community in the UK is small enough that personnel may well know who did the work.