I've been around a number of French touring/rando bikes from that period in original condition. They all had shellacked cotton tape. It is the French thing.
It's pretty clear from old pictures that French racing bikes of that time used cotton tape with no coating.
It wouldn't surprise me at all if there was some patented rubber tape or something from that time, but I've never seen it. Anyone recall Bailey (?) rubber tape from the 70s? I'd like to see some pics of these rubber grips mentioned. Were these like an early version of Grab-ons? Is it something different than the regular bar grips that the were often seen on drop bars pre war?