Hello Roy
Another lovely button.
In my view there is no difference date-wise between the backmark "C Jennens London" and "Charles Jennens London" (both with plumes). Usually, but not always, the abbreviated backmark is found on smaller buttons and I assume this was done simply for ease of engraving the master die on a smaller surface area and possibly depended upon the skills of the engraver. I believe both backmarks were in use side by side until about 1832 when the backmark became "Jennens & Co London" (and the use of the plumes was dropped for a period but that's another story!).
The fact that this button is open backed does place it in an earlier time period as most people agree that, in general, open backed buttons were being phased out by the 1830s. As with everything to do with buttons this is not a hard and fast rule and there are always exceptions!
So, given the backmark and the open back, I would say your button is almost certainly pre 1830s. There are some earlier examples of 1st West York Militia buttons shown in Howard's Militia Buttons book and they would date, I think, from the late 1700s to the early 1800s so possibly your button could be narrowed down further to the 1820s/1830s but that's just a guess on my part.
Roger
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