Hi there, and welcome!

I haven't heard of Diamondine before, but I looked it up on a razor date and manufacturing page, Straight Razor Manufacturers and Dates of Operation

I found these two on the list that refer to the name Diamondine.

HAMBLETON, W.B.
Import agency at 221 Spruce Street, Philadelphia, PA
Used trademark "DIAMONDINE"
ca. 1887-1888

VOM CLEFF, R
Import agency at 105 Duane St., New York City
Used trademark "DIAMONDINE"
ca. 1885-1926

Either one it is, assuming there aren't more, it seems that it's a very old razor! German made razor have a good rep, so it should be a good razor. Just make sure it's in good condition before you buy it. But I'm sure it'll make a good shaver. But just wait to see what the other users say, because I could be totally off!

Dave

p.s. What are you going to pay for it?