Beginning about 1892 it became a requirement for the chamber dimensions & weight of bbls to be marked on all guns receiving the optional Nitro Proof. As you stated the 70 is for 2 3/4" shells & the 20.6 (.811") is the dia just ahead of the rim seat in MM. You appear to have a good grasp on Belgian proofs & probably already knew this, but thought I would throw this in for those who perhaps do not.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra