Today at the Cambridge Glass Fair I had my hands on four of these "shot glasses" and checked them out with UV light.
Under longwave uv they show as a bright (but not "uranium bright") green and the shortwave uv result is (silvery) grey. For uv reaction, each glass was set upright and upside down as the fluorescence is barely visible in the sides of the glass but very obvious across the base and the rim top.
In all cases, including the examples that Malcolm and Martin have, the canes in the base are all known to be "early Ysart". Their weight, in the hand, varies and for two of the examples the glass was clearly thicker than the other two. Also, two appear to be unfinished as there are distinct "steps" in the upper parts / rim - but how that occured is a bit of a mystery.
For the four today, each had a clear glass dome above the canes but there appeared to be differing methods of construction. One had a dome part that seemed to have been made as part of the main glass and then completed with a "flat" section containing the canes. Two others appeared to have been made as "true domed" (but tiny) paperweights "inserted" into the base of the main glass. The other seemed to be a mix of the two methods!
All things considered my view is that these were most likely made at the Ysart Brothers (Vasart) works, 1946 to 1955/6. The UV result shows that they are not Vasart Ltd, 1956 to 1964 or Strathearn 1964 to 1980.
They could, however, be from pre-war Ysart years, made as friggers rather than part of the Monart output, but I lean towards Ysart Brothers.