My take on this is:
There is one set of black powder proof marks pre 1887, hence the
20B ?M Not For ball etc.
The next set is the decimal bore size,2 1/2" 65 grs. Black Powder
7/8 Shot etc. which is post 1955 but pre -metric.
The last set is the metric which looks to me like 16.3 on both barrels, 70MM or 2 3/4" chambers, and 950 Bar nitro Proof.

Better to keep your mouth shut and be thought stupid,than open it and confirm.