It seems to me to be reasonable to use as your primary unit of length the unit that was in use by that glassworks at the time, with an alternative measure given in brackets for those of us today who are not used to the system. So, for example, all British glass other than the most modern should be measured in inches, with a metric measurement given as an alternative if it could be useful to some readers, thus:- 8" (20cm). This provides the link to measurements in contemporary material such as pattern books, trade catalogues and advertisements.
It also keeps us well away from loony statements like "The Davidson Column Vase was made in three sizes, 15cm, 20cm and 25.5cm", when we all know it wasn't, it was made in 6", 8", and 10" sizes, and only serves to confirm that the writer is very confused, a complete plonker, or (in Britain) a trained archaeologist*, or, quite possibly, all three.
You can go one step further and use italics to show derived lengths rather than measured lengths, so
8" (20cm) would show that the object was measured in metric only, and that the inch measurement was derived.
So, my question is what primary measurement system should I use for Venetian glass dating from c. 1895–1920? Were the Murano glassworks using the metric system then, or some earlier system? It seems logical to describe the height of my blue vase as 13.5cm (5¼"), but I could be technically incorrect.
In a more general sense, when did the metric system become widely accepted and used on the European mainland? ... and what systems did it replace?
Bernard C.
* — Some years ago I saw the following written by an eminent archaeologist:
"Milestones are found along the road at intervals of 1610 metres".
Hard to believe, but it's true!