I believe Marples was labeling CF stones by at least 1860 and possibly as early as the 1830's. They didn't have them on the menu after around 1900 either.The other thing of course is that branding and labelling of whetstones is a relatively recent thing. If anyone can show me a stone label that definitely predates when Pike and WoAToS started doing it at the tail end of the c.19th, then I'd love to see it.
The quarrying of slate as a building material (and hence as a whetstone material) was a far more important industry in the middle of the c.19th century than it was by the middle of the c.20th. And the timeline of that decline also happens mirror the replacement of natural sharpening stones following the synthesis of man-made SiC and Al2O3, also at the end of the c.19th.
A lot of slate whetstones are just old, and came from a time when nobody labelled whetstones. And because slate was a very important building material, and it all looks the same... there are a lot of 'mystery slate' hones out there.