Clay, gun number 1849 was built on June 16th, 1871.

With your serial number of 1277 according to the Holland & Holland records, Clay your gun was built between 1865 and 1868.

According to 'The Shooting Field' on page 175, they state, that
"The details given at this early period are in most cases extremely brief, and in the remainder they have not been entered."

A John Stanton lock, that's not unusual. I was thinking either him or Joseph Brazier of Ashes Works, Wolverhampton.