Yes Lew! it is the “famous” 20 mm "Tube-Canon-exact denomination-" for the French Navy.
It was used to fire “economically” the Navy big guns…via a shhoting tube inserted and centered in the barrel, then safely blocked in place , to be strken by the gun usual firing pin.
An interesting point is the existence of a companion round, with a shorter case, proposed for the Army(Artillery) by A.Px, but not adopted, sometimes headstamped PUTEAUX in full l.
You will find more details on the ECRA Data viewer.
Another funny detail:
This naval ammunition was in use up to the late 50ies …! It even figured in the NATO inventory! Last headstamped cases were made by SFM as late as 1948, and so headtamped (4-entries)