Thank you all for the excellent help

@nigelc, thanks, I certainly agree it's a nice find. I was rather excited when I discovered it while examining an old dealers stock. It was among other 30c's and had gone unnoticed by previous owner

@perf12 - thank you for the link, I had not seen the 'gazette' in your last link. Very useful, it lists the 'Langer' on #69 (Type I). Upon further inspection on my stamp, I saw it was indeed a type I. It had been misidentified by previous owner and I was too preoccupied and excited with the company mark to notice. Classic bummer

@cjd - thanks for the Mercury Journal, very interesting indeed - that should be obligatory reading for all interested in classical stamps of France

And it confirms the same info as in the gazette. Brilliant.
Still it remains to find a catalog ID for the 'Langer' variety on #69. It's not listed in my Maury - I had hoped it was listed in Yvert Specialized ??