It was originally proved 1904-1925 (there is no 2 1/2" chamber length mark) with a bore 13/1 = .719" - .728"
It was London reproved (Crowned R next to the 850 BAR) 2001 now with bore 18.7 mm = .736"
For $14,000 one might expect wall thickness numbers

[Linked Image from photos.smugmug.com]

Gene posted some Riggs catalog pages here
https://www.doublegunshop.com/forum...p;Board=1&main=43449&type=thread