A trigger plate action isn`t a boxlock.....but it`s more a boxlock than a sidelock. I think that the auction houses `lump` them in with the sidelocks to indicate that they are of `superior` ( don`t ask me to qualify that ) quality to a boxlock - the same applies to W R droplocks.
The Scots made guns to compare with anything built in London - and usually at a lower price.
Gold washing of the lock parts is a particular feature of Mortimer and Harkom guns - but wasn`t consistently applied.
Joseph Harkom was taken over by Mortimer in 1875 ( I think that`s the correct date ) but guns bearing the Harkom name continued to be built until well in the 20th century. The distinctive Harkom Boxlock was developed under Mortimer ownership and is reckoned by many ( especially Scots ) to be the finest boxlock ever created. Mortimer also produced their own boxlocks which shared some of the characteristics of the Harkom - I have a 12 bore Mortimer shotgun and it is a beautiful gun.
How to tell a Harkom ? Look for the distinctive chiselled fences ........ just like the ones on the Daniel Fraser pictured above - that`s the third or fourth Harkom `Daniel Fraser` that I`ve seen.