In my neck of the woods I've seen a lot of old strops in antique shops, flea markets, those used by relatives, estate sales, etc... Almost every one of them were pasted.

The plain leather strops usually had paste on the "off" side. Those with canvas were usually pasted on the canvas. Some type of green paste seemed to be most popular, followed by black or red (in about equal frequency).

I wish I knew what the green stuff really was... (by brand name or manufacturer)