This is what seems to be the purpose-built successor model back then, photo supposedly taken in 1961:
If you look closely i think that's rather clearly a variant of an AMX-13 turret on the superstructure, thus switching to the in-service gun in the French Army at the time. One quad .50cal for air defence at the back and another two at the front. The mount in the middle of the back deck is probably an 81mm mortar.
Apparently the
Forces Maritimes du Rhin never had all that many of these heavily-armed vessels though. Possibly only two or three at a time, of different variants.
P.S. There are one or two other pictures where parts of the "Sherman boat" are somewhat visible, so it seems to actually have been in service to some extent in the 50s, and not just a one-off trials prototype.
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It should be noted that the
Forces Maritimes du Rhin existed for almost 100 years. They were originally fielded for the river-side defense of the fortress of Strasbourg in 1870, and having - in principle - similar barges with field guns mounted on them was introduced back then already (150mm field artillery guns back then). The squadron varied in size over time, typically between 300 and 800 men with around 30-40 boats of various sizes for various purposes, and mostly swelled during occupation times post-WW1 and post-WW2.
About half of the force was transferred to the nascent German Bundeswehr in 1957, much like its US and British counterparts at the time. What remained were the patrol forces along the stretch where the Rhine forms the border between Germany and France. In 1966 the Navy Squadron was dissolved for budgetary reasons and its tasks - mostly riverine patrol - and the boats for it transferred to the 32nd Engineer Regiment of the Army.
