I'm no alloy expert (let's hope one chimes in on this thread), but I read somewhere (
Alloying of Steels) that manganese steel is really a different type of steel.
I have a Bartmann razor that has "manganese steel" stamped in the tang. It's a very hard steel, taking more work to hone than most other razors, and keeps a good edge for a long time. In fact, I've honed it only once, so far and I estimate it has about 20 shaves on the counter now without any sign of edge deterioration. (Of course the hardening and tempering process defines a lot of the final hardness, so a Manganese steel razor from another brand might tell a different story) Based on the condition of most of those Manganese steel Bartmanns I saw on Ebay so far, I'd say it has some stainless qualities as well, but I'm not sure about that.
Best regards,
Bart.