Thanks gentlemen.
2 3/4 chambers by the gauge, not marked on the flats.
Could have been lengthened but I doubt it, on account of well-defined (short!) forcing cones.

...and agree about the age Clay:
I have always harbored the notion that the screw grip was too expensive to produce for boxlock shotguns after the 'roaring twenties', but may be wrong on that.
Will do a bit more arm-chair research when it arrives in my gunsafe.

Edit: Nigel Brown's Vol.1 gives the date range for the serial number block containing this gun as 1907 - 1912.

From memory, think it was Anson push-rod fastener, will confirm in the fullness of time!

Barry: you might be right about fitting a thick pad.
Trouble with routinely shooting buffalo rifles is that fitting a pad to a game gun didn't even enter my head!

...but a red pad would look good.