I'll get flamed for this, but a hollow in the length of a coarse stone is not going to be fatal for knife sharpening purposes. If you're going to use it for anything else lap it, but for knives, and especially ones with curved edges, there are probably more fun ways to spend your time.
Judging the grit is a matter of having something to compare it to. An old timers' trick is to run your thumbnail across the stone and get a feel for it that way. For the finer side you can tell a lot by the look of the bevel you hone with it: if it's a real mirror finish you are looking at 4000 and probably higher (only Turkeys and hard Arkansas stones seem to polish like this around the 4000 mark, and I don't suppose for a minute that's what you've got). If the stone is what I think it probably is, it'll be coarser than that. Any chance of a picture?
For knife sharpening opinions, try British Blades, or any of the other knife forums. Information overload of course.
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