The 18.7 along with the chamber length was the dia of the rear of chamber just ahead of rim seat. This converts to .736" which is a correct dia for a 16ga chamber. This mark was normally found only on those guns which also had the optional smokeless powder proof, which this one does, denoted by the PV mark.
It would be highly unusual for a gun to be chambered to a different length in each bbl, so one can mainly assume one of the marks was applied in error.

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