What's this acetylene bottle connection?

[Woods_Wanderer]

I'm considering buying this pretty cheap but I'm not sure what is going on with the tank connection for the acetylene bottle. It look like part of the assembly might be missing? What am I seeing here.

 

[dscheidt]

looks like a cga520 valve, common on 'B' sized cylinders (40 cubic feet, I think?).

 

[The Cobbler]

I agree on the B tank size, but it's missing the valve . not sure if you'll get anyone to exchange that as no one knows what's been let in to the tank, ie water etc...
I would put zero value on the tank. it won't even be easy to scrap because 1) it's a cylinder, and 2)being an acetylene tank it may contain asbestos