Monart labels may have been applied to all weights that were initially marketed out of the Moncrieff works - but most may have fallen off, or been removed. Many weights that Paul Ysart made may have been "marketed" by him personally, through visits to gift shops etc., without a Monart label.
It is not known whether weights destined specifically for the US, through Paul Ysart's deal with Paul Jokelson (started in the 1950s), would have had the Monart label - but it seems that most of the Paul Ysart weights known from that US trade deal do not have a Monart label.
I have 65 pre-Caithness Paul Ysart items (excluding "badge weights") of which 4 have a Monart label. I have probably seen a dozen or more others (in addition to Roy's) that have a Monart label.
My view is that the "extra value" attached to a Monart label varies according to who is selling it.