You've found about as much information as can be determined.

[Linked Image from photos.smugmug.com]

The chambers are 70 mm = 2 3/4"
At the time of proof the barrels weighed 1.400 Kg. If > 3% less now (from honing) they would be considered out of proof
The bores are 18.3 mm = .720"
The R barrel muzzle 17.6 mm = .693" so choke is .027" or Improved Modified
The L barrel muzzle is 17.3 = .681" so .039" or Full

The barrel flats will have the proof pressure - probably 850 or 900 Kg/cm2
900 kg/cm2 proof = 12,801 psi for a 600 kg/cm2 = max. service pressure of about 9500 psi

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