... genuine Paul Ysart creations but none have carried the labels complete with description - P/W No 15 for instance. If no description, I presume they must date post war and be 1950's fare?
I would not use that as a means of deciding when a Monart label was used on a paperweight (or bottle).
As far as I know, there is no evidence of who actually applied Monart labels to PY weights and bottles and whether it was always, sometimes or rarely done. But it is probably a reasonable thought that it would have been done by Moncrieff's shipping clerk, or similar person, using a coding created by Paul Ysart or maybe just the shipping clerk of the time.
There is also no evidence, other than the labels themselves of the categorisation of weights as "P/W No. <n>". The info on the Ysart Glass site is simply constructed from those labels that have come to light.
All of the Monart labels I have seen on weights or bottles have been of the "Monart Glass" type (as opposed to "Monart Ware"), but without the "shape and colour coding" variations as for vases etc., it is not possible to say that a Monart Glass label on a weight or bottle is pre- or post-war period.
As for the current eBay weight mentioned, I agree with Sophie that the look of the ground and likely clarity of the dome suggest 1950s. But a similar Monart label could be found on an item from the 1940s - as with my scratch signed, dated (1946) and labelled millefiori bottle