Just for confirmation, are the "letters" those seen in the second photo, taken in a horizontal view, showing the "stem" below the central air bubble? If so, then I agree with Frank.
This type of "coincidental occurrence" can occur in many situations. Although "M" and "N" shapes do seem to be clear, these and the other "marks" in that area are simply gaps between the white glass.
To add to what Frank has said, this weight has been made with lots of coloured and white glass bits, but probably few, if any, actual "canes". Paul Ysart usually put the coloured and white bits (and often with chips of gold aventurine, too) on a surface, which was formed from powdered white glass. When the tool was inserted to form the bubbles over "legs" / "stems", the powdered white ground gets stretched. If the powdered glass was quite thin in any area, then the stretching will cause gaps to appear, and, in this case, it has given the allusion of "letters".
Having said that, I will now add that I think it is a very good paperweight and it is also less common than many other types. Here is some background information ...
I have two similar weights, one is a double Harlequin (like Henry's mum's) and the other is a Triple Harlequin (as shown here
). My Tiple version is much like Henry's mum's in that is has no clearly seen whole or part millefiori canes. But my Double example does have partial canes that are matched to others in Paul Ysart's work.
So far, I have seen about 6 (or less??) of these weights, all being of large (magnum) size and with a smooth polished base. I believe these are from the 1930s period, but they could also have been made post-war, prior to Paul joining Caithness.
At Caithness, Paul made some Double Harlequin weights but when asked by Colin Terris whether a Triple was possible, three were made. This information is still available in one of Colin Terris's websites (although Colin is sadly no longer with us). See this link
then click on "Patience Rewarded". Unlike my two and Henry's mum's examples, the Caithness version is much more like the Harlequin types that Paul made at his Harland works in the 1970s, having coloured twists amongst the cane sections and coloured chips.