Mike, 20674 would have been serial numbered in late 1876, or very early 1877.
It was sold at Bonham's auction on 27 July 2005,
https://www.bonhams.com/auctions/11775/lot/3/then at Gavin Gardner on 15 November 2023
http://gavingardiner.com/BidCat/detail.asp?SaleRef=0049&LotRef=63It has steel barrels which probably means it was re-barreled at some point. Nobody in London at that time I don't believe was using steel barrels even though the Whitworth patent was dated to 1865. The earliest Reilly ad for Whitworth barrels was in January 1882. There is a Reilly pigeon gun dated to December 1881 with the steel barrels. Here is the grain sheaf trademark:
This said there is a Reilly double rifle .500 BPE with 28" barrels SN 19953 (1875) which sure looks to have steel barrels.
Check out the Reilly History:
. . -- Chapter XI: Reilly – 1880’s:
. . . .-- sub-para 76. 1882: Reilly and Steel Barrels.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=436538&page=95I'd be very interested in knowing if there is an address on the rib or barrels and if there are the Whitworth grain sheaves on the barrels. If so then this is going to be a revelation.