Perseverance has paid off.
Many thanks to AGL Historian Karli Dunstan.

I have received the "birth record" for my Stephen Grant .577 Express.
Was originally 2'6" barrels (30") so barrels have been shortened to 26" some time later.
The action came from "Scott, as a 2nd Quality Action" (W&C Scott?) and the rifle was completed at the Grant workshop 67A St. James Street, London in 1873.

Original owner was one Thomas Algernon Smith-Dorrien-Smith
https://en.wikipedia.org/wiki/Thomas_Algernon_Smith-Dorrien-Smith

- Mike