Identifying Antique Hones
Great that your 12K hone is working well for you. Unlabeled stones can be hard to classify, especially true for the grit level of natural stones. Someone here may be able to help if you can provide any label info that may exist, dimensions, and good photos. You might try out your grandfather's stone and some sharpness tests and a test shave to see what happens.
I mostly work with pasted bench strops and suggest also giving consideration to trying CrOx and diamond spray/paste on honing leather etc. Bench strops are easy to make and inexpensive. I hope that you get additional responses as the date of the post before yours is September 2009.
HTH