1900 means it could have been originally proofed for black or Nitro and for that I would need to see the proof marks to be sure. In this period if nitro it usually had the shot weight plus the word 'MAX' . I would guess it originally had 2 1/2" chambers as the re-proof was at 850 bars and will have the 65mm. mark. If it had been 2 3/4" chambered then they would have re-proofed it at 70mm. length automatically. If in bars it would have been re-proofed post 1989; the 1954 rules of proof had the chamber length in inches and pressure in Ton per sq. in. Factory records won't probably tell you much other than when it was made. If you have the serial number I can date it to the year for you.
Most likely it would have been 13 over 1 and could have been opened out to remove any pitting prior to re-proof and should now have the new bore diameter stamped on the barrel flats. It could have been 15 over 1 and still be chambered 12 bore but the bores would have been tight. Not impossible but improbable. Hope this helps. Lagopus.....
