You have the marks deciphered

[Linked Image from photos.smugmug.com]

The original bore was 13/1 = .719 - .728"
Reproved after lengthening the chambers and honing to 18.6 = .732". The reproof date code is in the upper right, but I can't read it.

As to service pressure, it's complicated frown
The CIP standard for 12g 50mm, 65mm, and 70mm “Standard Proof” lead or steel is 850 BAR (Crown over STD)
Maximum Average (SERVICE) Pressure 740 BAR = 10,733 psi;
Maximum Statistical Individual Pressure 850 BAR = 12,328 psi

The British 70mm chamber standard was to a service pressure of 3 1/4 tons/sq. inch or 900 kg per square cm (BAR)
900 BAR is for a “Maximal Average (Service) Pressure” of 780 BAR = 11,313 PSI

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