66 Block

The most common type of connecting block used to terminate and cross-connect twisted-pair cables. It was invented by Western Electric eons ago and has stood the test of time. It is still being installed. Its main claims to fame: Simplicity, speed, economy and space. You don't need to strip your cable of its plastic insulation covering. You simply lay each single conductor down inside the 66 block's two metal teeth and punch the conductor down with a special tool, called a punch-down tool. As you punch it down, the cable descends between the two metal teeth, which remove its plastic insulation (it's called insulation displacement) and the cable is cut. The installation is then neat and secure. 66 blocks are typically rated Category 3 and as such as used mostly for voice applications, although Category 5 66 blocks are available. 66 blocks are open plastic troughs with four pins across, and the conductors are more susceptible to being snagged or pulled than conductors terminated on other types of blocks (e.g., 110, Krone or BIX).

A note on the Bell Labs numbering system… They just started with "number 1" on whatever system they were working on. TD1 radio, TD2 radio, etc., Whenever there was a "hole" in the sequence, that meant that the labs had worked on something, but it didn't pan out for some reason.