Back to the question about chamber length. Because the Brits typically use the .800 plug gauge to measure chamber length, there can be a problem--to which I believe Miller was referring--if a British gun has its chambers lengthened by a gunsmith using standard American 12 gauge reamers. Our 12ga standard is .798. Therefore, if an original 2 1/2" Brit gun has its chambers lengthened using American reamers, the .800 plug gauge would still read it as 2 1/2". Something like the Galazan chamber gauge, however, would read it as 2 3/4".
So, in the case of this Greener, if the owner used a Galazan gauge to measure chamber length and came out with a measurement of 3", it's quite possible that the gun started life with 2 3/4" chambers and, at some later date, had its chambers extended to 3" in this country--obviously without British reproof. The easy way to tell if they were ORIGINALLY 3" would be to use the .800 plug gauge.
