Is it possible that these original copies of mills manufactured by the French were distributed to the French troops?
It is possible that some were trialled by the French Army.
The order placed directly by the British Army with a Monsieur Charles Vandersluys, a Dunkerque copper boilermaker, was for grenades solely for the British Army. However, while the shortage of Mills grenades was acute in mid-1915, one possibility is that the British Army passed some of the French-made Mills grenades to colleagues in the French Army for their evaluation.
French production was terminated after 180,000 grenades had been made, when Vandersluys and the companies he had sub-contracted were threatened with legal action by the representatives of the Compagnie Belge des Munitions Militaires (CBMM), claiming patent infringement of the Roland grenade (invented by the Belgian soldier, Captain Leon Roland).
CBMM, which effectively owned the design of the Roland grenade, had failed in its efforts to get the French Army to adopt it, probably for the same reasons that the British authorities had not adopted it: it was too dangerous and unreliable.
William Mills had taken the Roland design and developed it into something less dangerous and unreliable, which became the Mills grenade. Another possibility is that CBMM handed over some of the confiscated Mills copies from the French firms to the French Army for trials - if the French Army had adopted the Mills copies, French law would have allowed CBMM to manufacture and sell them as a modified Roland grenade.
Somewhere there may be documented evidence for these possible events, but without the evidence the explanations of Mills grenades in French Service markings remain purely speculative.
Is it possible to get a few photographs of the blue Mills? That would be very helpful.
Tom.