20 gauge is one of the most difficult to recognize.
- In 1961 (1969) the new CIP standard managed , not only to decrease the minima for head and base diameters,but furthermore to give to the maximum of the rim the minimum dimension of the chamber (with zero tolerance)
for 20 gauge, CIP dimensions (before 1961) are
Rim diam : 19.25 - 19.40 mm
Head diam : 17.60 - 17.70 mm
Rim thickness :1.40 - 1.50 mm
and after 1961:
Rim diam : 19.20 - 19.40 mm
Head diam : 17.55 - 17.70 mm
Rim thickness :1.35 - 1.55 mm
- Therefore you have to determine first the date : before or after 1961.
If it is before 1961, it is still very difficult for a 20 gauge.
Indeed the French standard was changed (only for the 20 gauge) in 1930 to be compatible with the CIP dimensions !
Like you can see with the following tables , except if you are lucky to find a ctge with a dimension fitting in one categorie and not the other, it is not easy.
French dimensions (standard 1930)
Rim diam : 19.20- 19.40 mm
Head diam : 17.50- 17.70
Rim Thickness : 1.30- 1.50 mm
Leon Beaux (standard 1947)
Rim diam : 19.10 -19.30 mm
Head diam : 17.45- 17.60
Rim Thickness : 1.40- 1.50 mm
In any case your rim dimension looks to be too small .
(It is very difficult to measure specialy when the rim is conical. I use go no go gauges)