Hi Judith,
Comyns registered his first
mark in 1859, the firm continued as William Comyns & Sons until sometime in the mid twentieth century and used very similar marks throughout its history. From your photo, it isn't really possible to tell which mark it is, but from the style it would appear to date to the late 19th or early 20th century.
There are a few troubling things about the marks; it should have a date mark (and duty mark if pre 1890) and the ring around the lens should also carry hallmarks, if only a partial set of them.
The fact that is has not a complete set anywhere and is missing some that were technically required is generally indicative of a made up piece. It is not an uncommon practice to make a pretty and functional piece from less saleable items such as carving set handles and buttonhooks. This may not be the case with your magnifying glass, but it seems a strong possibility.
Regards, Tom
.