CJF;
Has this gun possibly had the barrels opened to a 20 gauge from a smaller rifle. That 39 would seem to be a "Gauge" mark. 39 gauge equates to .492" in diameter. This would have been marked per bore diameter, not including rifling grooves. Did it perhaps start life as one of the .50 caliber black powder rifles?
Offhand I do not recall the date that British proof houses discontinued using the gauge mark on rifles & went to actual measurements.

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