There seems to be great confusion about the diameter of the UK silver florin between 1893 and 1970.
Some sources, including the Royal Mint shop and the Numista website, indicate a change of diameter in 1937, giving 28.30 mm for 1893-1936 and 28.50 mm for 1937-1970. Other sources give a single (but different!) diameter for all florins between 1893 and 1970: Krause (28.3 mm), Wikipedia (28.5 mm), Peter Davies (28 mm) and Matthew Dickinson (28.5 mm).
Analysis of the coins in my collection refutes the idea of a small increase in diameter in 1937. On the contrary, there is a statistically significant decrease in diameter. I find no evidence for the existence of a florin measuring 28.30 mm of any date.
For 1893 to 1936, the diameters measured range from 28.44 to 28.74 mm. From 1937 to 1970, from 28.36 to 28.57 mm.
Assuming that the Mint measured in inches, my conclusion is that the correct (nominal) diameters for florins are as listed below:
Godless 1849: nominal diameter 1.10 in = 27.94 mm (rounded to 28 mm)
Gothic 1852-1887: 1.18 in = 29.972 mm (30 mm)
Jubilee 1887-1892: 1.16 in = 29.464 mm (29.5 mm)
1893-1936: 1.125 in = 28.575 mm (28.6 mm)
1937-1970: 1.12 in* = 28.448 mm (28.4 mm)
*Another possibility is that the diameter was “metricated” in 1937 to 28.50 mm
Unfortunately the Coinage Acts give only the weight and fineness of each denomination, and not the diameter.
Any thoughts on this would be gratefully received.