It's usable, and it looks to be a good stone from how much use it seems to have had in its past life. You just have to decide if you have the patience to true it up (flatten it) before you put a blade to it. As far as the manufacturer, there's no way to know for sure without a maker's mark. I find stones like yours at garage sales & flea markets here, many of them in worse shape than the one in your pics. I have fifteen so far, maybe more. I have at least six or seven I haven't trued up yet. The first thing I do is set them on my glass plate, with the worn side down, and measure the wear with a standard ruler. If the "dip" is too deep (more than a quarter of the material depth for that side of the stone), I don't bother with it. The second thing I do is soak every stone I get for an hour or two (sometimes several soaks) in hot water to get rid of the oil in them, especially if they're gummed up. It's hard to tell how coarse a stone is when it's never been cleaned. The other thing is do NOT try to get rid of those rounded-off edges. From my experience, you'll only make it worse, and end up with an ugly paperweight that looks like an anorexic brick. (Don't ask me how I know, I refuse to comment.

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I had a flat steel plate I used for years with silicon carbide grit & water - I originally got the idea by watching a guy in our cabinet shop flatten plane bottoms like that. Then I dropped it during a move and cracked it :grumpy: so I recycled it. I now have a piece of plate glass that I use with the same grits (wear nitrile gloves!), and I checked it with a steel machinist's straight edge last year and it's still
almost perfectly flat. For the stones in the worst condition, I just start by scrubbing the stone on our driveway (you just need a flat spot), then move to the glass with the grits (or emery cloth) & water, and work my way to a finer grit. If the stone has a deep dip in the middle, don't push downward to hard, or you risk having one semi-useful stone turn into two useless ones instantly. You can seriously screw up a good stone, so go slow.
There are products like "Eze-Lap", but they can be pricey if your only flattening one or a few stones. I've had varying degrees of success with things I use, so I haven't bothered spending money on any of those products, other than the silicone carbides grits.
If you really want to get this stone into workable condition, give those things a try, and if nothing else, email me and I'll send you my address, and I'll see if I can get it flattened for you.
~Chris