I have found an old Carborundum hone - how do I determine the grit.

I think it can be quit old - belonged to my grandfather so it can be between 20-50 years and its very well used. The bottom was flat (luckilly) so it is possible to use it

I honed a razor I recently bought and it did not take me long to get rid of a relatively big chip...so it is fast.

Any ideas? - I can try to post a picture later on.

Thanks
/Henrik