The SN dates the DOM to 1903
I've not seen a Francotte side selector.
Here's a sideplate boxlock with a Greener side safety
http://www.safarioutfittersltd.com/EuropeanPage47.htm
At the time of proof the bores were 18.4 mm = .724" and the Belgian typical
Right is ?18.2 = .717" = .007"
Left 17.7 = .693" = .031"
I don't see that the chamber length at proof is indicated; and that was part of optional Smokeless Proof.
It would be marked as 70 (if 2 3/4") over the chamber diameter, usually 20.6 mm for 12g.
Lots of Francotte imports though are marked 2 3/4" on the underside of one barrel.
Threaded Mode