Curl:
Regarding year of manufacture, my Alex Henry 20/.577 ULH DR (SN #5514) was finished on the 6th of November 1882. The only other AH .577 I have seen (also a 20/.577!) is SN #5860 which was finished in September 1885. It would seem likely that yours is very late 1880s or even into the '90s if the SNs were sequential. Do let us know if you send off for the journal extract.

BTW, both those rifles are very similar to yours, especially #5860 which left Australia in March 2001 and is now owned by Frank Findlow in the US. #5514 is a slightly higher grade, judging by the fully scroll-engraved sight-leaves and a few other minor details, but is rather down in condition compared with the other two (especially yours!).

Re-read the article on #5860 by Frank Findlow in the Summer 2005 Double Gun Journal while your shoulder readies itself for action!

"A Scottish Enigma: Alexander Henry's 20/.577 Express"

Apart from the slightly different chambering, it's your gun, mate!