Raimey,
That's not an "imported tube" stamp, that's a "proved in the finished state" stamp.
The Manufacture d'armes et cycles de St Etienne used it all the time and they certainly were making their own tubes.
This stamp was used to denote the fact that there had been no provisional proof of the tubes. The maker was confident enough that the tubes were OK and finished the gun without provisional tests. It was all OK with the proof house as long as the gun passed the final proof. If the gun did blow up, money would have been wasted, but that's not a problem for the proof house. Companies with tight quality control and good processes don't need provisional proofing.
There is no such thing as an "imported tube" stamp at least in France.
There are stamps such as AE palm, AR. ETR. or ARME ETRANGERE, which are stamped on foreign weapons.
Granted, if tubes had been imported, the gun could have borne a "proved finished" stamp. However, that's not automatic, far from it. Besides, Belgian tubes would have been proved in Belgium anyways.
Best regards,
WC-