I didn't know it either until I happend to put a magnet against both to determine what might be in them. BTW, speaking of early barber hones, their is a VERY close relationship to the present Norton-Pike (Saint-Gobain) comany and the Pike I have with the fish swimming though the capital "P". Norton Pike History
I have a Norton HS4 transluent slip stone and it's seemingly more fine that simply 4000 grit. Almost glass smooth.