Extremely rare Antique CELTIC Choker Necklace, carved Bog Oak and Brass, Probably Victorian

An amazingly rare find. A choker made up of 7 brass medallions each with CELTIC artwork inspired by the illuminated manuscripts such as the Book of KELLS.
The brass artwork is set into what I presume is BOG OAK which is also intricately carved with Celtic knot work. This is a piece of art in its self and something unique to treasure.
There is a maker’s mark on the back but I’m unable to locate its origin. It is a diamond shape with the possible letters of R in the middle and I, S and M surrounding it.
I’m presuming this was made sometime in the 1800’s.
Dimensions: each medallion 32mm diameter, Choker length 35cms