I have a 1927 Specialty Grade set of barrels with the Trap options and the right barrel is .725 and the left is .730 so not much different than yours.
Most of the choke length starts at about 2 9/16" with the last one inch being parallel.
Measuring the outside of the barrel for wall thickness is really useless, you need a barrel wall thickness gage. It will give you readings from the top rib to bottom rib.
Your 12 ga. is from 1897.