Morse's first insulator, devised by Cornell, as written up in various accounts, was 2 grooved blocks of glass, with the wire sandwiched between them, set into a slot cut in a wooden cross arm and with a block of wood nailed over top to hold it together and keep the rain out.
This block of glass, came from an 1840s privy along the route of the 1847 Erie & Michigan telegraph company, a Morse line. While fairly primitive, it does show design features suitable to be used as an insulator; central groove, flared ends, and a side projection that would keep it from slideing out of a cross arm or wooden block. Another block identical to this would be laid on top and this would hold the wire sandwiched between them.
1847 would seem late to still be using the "2 blocks of glass with the wire bewteen them" form of insulator but the Erie & Michigan Telegraph Co. was a poverty stricken line when constructed (where incidentaly Jeptha Wade would get his introduction to the telegraph business). Cornell's letters, availbe for reading on line, speak of great hardship, no cash, no credit, the line was built hand to mouth. The early block insulators may have been all that was available or they may have had some practical application, like being used where a wire had to enter a building..