Many thanks to all of you for the advice and the courtesies that you have extended to me.
calibre 60 bore (.450) per factory letter
Groove diameter .458
twist 16
the following from Purdey:
" 1891: Nwe 450 barrels fitted andreshot with 450 (3 1/4) solid drawn case. 3 3/4 drams and Eley papered bullet."