I'm finding Mannesmann to be a producer of seamless tubes.

They date to 1890 or so, and apparently use a variation of rotary forging.

I find no reference to them as a steel maker.

'Acier Mannesman Special' would seem to be an embellishment along the lines of 'Fine Corinthian Leather'. It's essentially meaningless.

Again, caution. The design and construction of this piece is highly suspect.

"The price of good shotgunnery is constant practice" - Fred Kimble