Drew,
As I think you know, I've been studying Rigby and Powell records for some time. In the case of Rigby, a "barrel book" tracked the source of their tubes. Each page has descriptions of 12 barrels. A tally of two such pages from 1893 has one barrel from Vickers, two from Whitworth and thirteen from Arinen (sp?) - all of which were steel. Damascus tubes included four "plain quality" and four "Crolle 4 Stripes" - all from HL & Cie.
If Powell had a similar book it has been consigned to the 'great gun ledger grave yard in the sky." However, notations concerning barrel material were noted in their day books. They first used "punched Steel" in 1867 - but only for a few years. Damascus tubes were described as Iron Damascus, Stub Damascus, Damascus and Best Damascus. Rifle barrels were made from Laminated Steel.
Whitworth Steel appeared in 1881 and Siemens in 1886. Additionally, I believe Powell was using domestically produced Damascus for most of their barrels because begining in 1885, occasional notations were made that the barrels were from Belgium.