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Than its a synth fine hone, glued to slate. I bought one like that too, thinking it is a coticule. Mine wasn't the best for straights, it was around 4-6k in fineness.

If the stone is 40 years old, than I am fairly positive that it will have a bbw backing, IF it is a coti. It looks like one, but I have seen some synth combination stones that looked deceivingly like...

Could be a coti, especially if it is 40 years old. Does the lighter side soak up water? If you scratch it with your fingernail, does it feel totally smooth, or does it feel rough, or has any low...