I did some searching on the web and on my computer files to illustrate the various high quality locks seen on British work, primarily with an interest in showing cosmetic variation. All of these can be dated from the late 1850's to the late 1860's.
Typically, these locks are fitted with a safety bolt either fore or aft of the hammer. Most often the safety bolt engages a mating mortise on the inside of the hammer. Occasionally, an additional simultaneous mortise locking the tumbler is seen but that is rare on this later work. I am not sure why Mr. Rigby opted to omit the safety bolt on his target rifle locks but they are often seen on his sporting rifles.
There are two basic styles of 4 screw bridals most often seen but these differ visually only in how they were decoratively filed out. The screw patterns however are unique to each style. Less complicated 3 screw bridals may have been used on occasion for 'Best Quality' work, but I have never seen one.
I once had a fully assembled unmounted 4 screw lock of this period from the collection of Ian Crudgington. The lock was completely functional but unpolished with a safety but had no hammer. The bridal was literally a flat plate having not been yet decoratively filed. No draft had been filed on the somewhat oversize lock plate. This is what we would call a gunmaker's lock today, probably built in the Wolverhampton area for the gun trade. One of these locks would explain an unusual variation of style.
Here are the two most commonly seen bridal patterns. A Stanton lock first and a Brazier below.
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Here are some photos of various makers locks.
A Charles Ingram Match rifle.
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A Edge Match rifle.
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A Rigby Match rifle. Note absence of safety.
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An Alex Henry sporting rifle. The link below allows good resolution and manipulation of the images.
http://www.bonhams.com/auctions/17944/lot/329/Lock mortise of an Alex Henry Match rifle illustrates bridal configuration.
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Jim Westberg