I would think a replacement but Running Man has looked at billions of pictures of Russians so I would await his word on the missing Tula symbol on a true 1955.
Hey now, let's not conflate numbers. It's way, way less than that! Only about 5k or so Russian SKS photos that I've got saved!
I've generally assumed that they did away with the stock cartouche sometime in late '55s. The stocks are too uniform and from too varied of sources/importers to be such a consistent refurb.
It's clear that the by the time they were producing letter Д guns, the stock cartouche was definitely gone (well unless you buy a Westrifle imported Canadian gun, they have 'original' cartouches all the way through 1958!
Other examples of CM prefixed '55s: