I've only scanned a source or two and can add very little but Gye & Moncrieff was a partnership that peddled longarms from 60 St. James's Street, London from 1876 to about 1885, when they appear to have moved to 44 Dover Street and dropped off the map in 1887. I haven't seen one or searched for the patent(1878??) but they designed some odd breech extension lock-up to compete with the Purdey lock-up with interfaced with the lumps of the tubes.

Alexander Moncrieff of Perth, Scotland?
http://www.google.com/patents?id=sYtUAAA...;q=&f=false


http://books.google.com/books?id=lpsEAAA...don&f=false

http://books.google.com/books?id=euANAAA...;q=&f=false
Kind Regards,

Raimey
rse