In theory, there were 2 different styles. One was a lever type and one was a push button type.
The lever type was the most simple, typically used on a S1STD. The push button type when used on a S1STD had a mechanical/pneumatic interlock that fit on the FWD/REW switch so it would not work when the spindle was ON.
The push botton type on the CNC usually tied into the air line to the brake cylinder (BOSS 8, BOSS 9, V2XT, some BOSS stepping motor CNCs) and had a pneumatic delay which allowed the spindle to stop and then the drawbar to operate.
The older CNC and EZTRAKS that did not have the pneumatic brake used a variation that had a electric solenoid that would disable the draw bar if the spindle was turning.
These extra parts raise the price. See exactly what version he is selling.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)