The CV lenses do carry serial numbers. On my 15mm M, as well as 75 2.5 LTM and 90 3.5 LTM, they are all on the same location.
That is, at 6:00, directly opposite the focusing index / depth of field scales, on the undersurface of the lens when mounted. It is an engraved number on the body of the lens, just underneath the focusing ring. Since the engraving is very shallow and not in any contrasting paint or chrome finish, you may need to use angled lighting to see it.