As part of a project with a Wingnut Wings RNAS Sopwith Pup I just started, I have a CSM figure which would go with it. I was looking into the life vest the figure is wearing, and can find no references to what I'm seeing sculpted, although it's close.
Finding photos of an original on an Internet search, and also an illustration, it seems the large cylindrical tube on his left side may be a gas cylinder used for inflation. But the original in the photo has the cylinder mounted with the valve on the bottom, and the top tapers down to the edge of the vest. Perhaps there were variations?
Cheers!
If anyone can shed any more info that would be great. My guess right now would be to taper off the top of that cylinder and contour the sides as per the photo of the original vest.

CSM figure with life vest (and large cylinder on his left breast)

Illustration showing cylinder on pilot's right side

original vest from a Internet sale showing inflation cylinder on vest's left side and valve at the bottom