Terc,
The bbls should be marked what their original nominal bore dia. was at time of proof in mm. Trust that to be a good number. Do the conversion, is that .621? If so, then the bbls are unmodified, in that regard. If not, I have to ask myself how likely it is that some one would have back bored them. Choke markings had gone to the asterisk(s) by 2001, but they too should be on each bbl. If it is a 3" chambered gun, it will have the higher definative proof marking of two stars inside a circle over 'PFS' & if a 2 & 3/4" chamber may have only one star inside a circle over 'PSF', which would be the normal final proof for 2 & 3/4" .
I'm going to assume that you meant to type that the right bbl. has .010 & the left .020 constriction, which would make it IC & Mod., though it may pattern tighter than that w/today's ammunition.
Easiest way to tell if the bbls are modified is to compare their present weight to that stamped on them. It will be in KG and read something like 1.XX Kg. You can convert it to pounds.If they have been back bored it will be easy enough to tell from weighing them and making a comparison to what is stamped on them.
All the above says that you likely do not have an issue aside from double checking that someone has not indescriminately lengthened the chamber or forcing cones from 2 & 3/4" to 3" or back bored them.
I'd guess that the reason it remains in almost new condition is that it was found to have a rather abrupt 'thump' by its original owner using std target loads. If they shot some 1oz. field loads in it if it is a 2 & 3/4" gun or God forbid, some 1 & 1/4oz. 3" rounds(!)there is little doubt where the incentive may have come from to let somone else enjoy it.
Also, you sometimes have to take other people's measurments w/a grain of salt. They do not all know how to read nor use measuring instruments w/good accuracy. No offense intended toward anyone, just saying.
