A G grade Lefever in the 21xxx serial number range will most likely have Damascus barrels. I have never seen a G grade with twist barrels, but it is a Lefever so I will never say never. I would also expect 2 5/8" chambers, but if the gun was ordered for use as a heavy waterfowl gun, and the 32" barrels are a clue, then it is possible the customer specified longer chambers.
Your gun was likely built in 1893, so it could have had any manner of alterations in 126 years. I would try to find someone with the tools to properly measure 12 gauge bores and chamber. But remember, a fired shell is not a chamber gauge, and many times a fired 2 3/4" shell casing will slide fully into a shorter chamber without much resistance, especially if the forcing cones are not abrupt. This is also why the frequent suggestion to use a rolled up 3" x 5" index card is also not a great choice. And even those fancy tapered brass chamber gauges may go in further than true chamber length if the chamber is a tiny bit oversize in diameter. I agree with Miller on the simple use of an inexpensive 6" stainless steel machinists scale or ruler. They aren't very expensive. I probably have 10 or 12 of them, and found most at flea markets or swap meets for $1.00 or less. They have graduations in 32's and 64's of an inch, so are more than precise enough. You simply set the barrels in a stable position pointing toward a light or sunny window and look into the chamber. You should easily see the transition between end of chamber and start of forcing cone. Slide the scale into the chamber until the end reaches this transition and read the length at the breech end. Many of these scales have a sliding pocket clip that can act as a stop to more easily get a reading without disturbing the position as you raise your head. Double check your reading and check both chambers, and you're good to go... as far as knowing the chamber length.
As far as having been reamed out and unsafe at any speed, only someone with proper wall thickness measuring tools can know for sure. But in my experience, odds are everything is original unless there are obvious signs of reaming and honing. Most of these guns, especially 32" models, had pretty heavy wall thickness to start with, so even if the chambers were opened up to 2 3/4", it probably wouldn't make the gun dangerous. But starting off by shooting a 126 year old Damascus double by firing 3" mags is only more likely to damage the old wood than to blow the barrels. You obviously know better than that. Get it checked by a competent smith with the proper tools, and enjoy it with appropriate loads. Or sell it to someone who will.
