It must be a very old plug. There aren’t very many still around (in France and Belgium anyway) that aren’t the hybrid type which fits either Schuko or the Belgian/French outlet. The only exceptions being some rewireable plugs.
Incidentally, that standard originated in Belgium, even though everyone calls it French. The old French grounded plugs were 3 pin. Seems it was adopted by Poland and Czech Republic, Slovakia (then Czechoslovakia), as it avoided paying royalties for ‘Schuko’ and is theoretically polarised.
That plug / socket system seems to have been developed by Vynckier in Belgium, a company that still makes extremely good electrical equipment and is now part of GE.
It’s defined as CEE 7/5 (Socket) and CEE 7/6 plug.
There are only three CEE 7 plug types sold these days and they fit both “French” and “Schuko” outlets.
CEE 7/7 - Grounded 16 amp
CEE 7/16 - Flat ungrounded “Europlug” for small appliances up to 2.5 amps
CEE 7/17 - round non grounded 16 amp plug (common on things like vacuum cleaners and hairdryers etc)
CEE 7/16 is also compatible with the country specific outlets in Denmark, Switzerland and Italy as well as old obsolete types. It will basically fit anything in continental Europe as well as a lot of outlets in Israel, even Chinese 2 pin sockets tend to fit it.
The rectangular 3-pin system used in the U.K. and Ireland is the only one that isn’t compatible.