Raimey;
I for one have throughly enjoyed this exchange & feel I have learned a lot from it. I had wondered on occasion just how bores smaller than 172.28 (.300"/7.62mm) were marked during this era but had not had opportunity of examining one.
Interestingly doing the conversions on these reveal;
181 = .2951"/7.5mm)
184,11 = .2934"/7.45mm)
To have followed the system adopted from the British with the larger sizes & dropped an even .010" to .290" (7.37mm) would have given a plug of 190,72 gauge.

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