For some reason I seem to recall some funny math in the 950 bar conversion. But it sounds the subject longarm began as a 32 bore and was opened up to a 28 bore. The crown over R possibly notes the original proof was black powder and the current is smokeless, possibly along with some alteration. Are the twist tubes Belgian? Can you post or email a pic of the flats? What are you wall thicknesses? Has it been sleeved?

Kind Regards,

Raimey
rse