Doug,
I dug into my library last night (I apologize for being a bit of a bookworm) and found out a bit of information from Neal and Backs book on the Packington guns. There are 3 forms of the linkage that are known on English guns. The first is found on a lock built by Thwait of Bath England around 1760. The linkage is quite long because the upper tip of the feather spring dips down dramatically toward the bottom of the lockplate. It is pinned to the frizzen only and the bottom of the link fits into a groove in the spring. The camming action had a long arc to travel through. The second form is found on a gun by Griffen and Tow from 1779. The link is attached to the frizzen but has a simple stirrup on the other end that slides on the spring. The third form found on numerous guns such made by Twigg, Nock and others is the style you have on your lock. Nock used the device on some of his volley guns as late as the mid 1780s. Apparently the roller bearing on the frizzen or the spring appeared at about the same time as the Thwait lock and both devices were built for a while. I guess the roller bearing won out, probably because it would have been much easier to make. By all accounts the device worked very well.
dave
