Looks like a coti, I have found a few like that with the bbw dished like that. It won’t be hard to lap flat with some 60 grit wet & dry or Silicon Carbide powder, finish on a DMT or 220/320 wet/dry.
It probably belonged to a butcher or commercial fisherman, saw one of those commercial fishing shows the other day where the guy was going to clean some fish, he pulled out a big coti/bbw, slapped the coti side down and sharpened his filet knife on his dished bbw side then gutted his fish. That stone is probably older than the dude.
Oh well, it’s just a tool. Good find, they are out there.