Western Electric Keysets with exclusion turnkeys.

Most keysets have pushbuttons for hold and line selection. These sets have a turnkey in the rightmost position, which may be used to provide line exclusion, ringer switching or other functions. See the BSPs for detail.

If you look close, you'll see the knob on the 440 is knurled and has a vertical white mark to show the switch direction. The later sets have square buttons, so only have a thin black line to show the presence of the turn switch. These are fairly scarce and you have to look close while wandering through the phone show or flea market to pick them out. Happy Hunting!

More on Western Electric keysets can be found on these pages:
http://www.paul-f.com/we300typ.htm#Keysets
http://www.paul-f.com/we500typ.htm#Key_Sets
http://www.paul-f.com/we1500typ.html#Keysets
http://www.paul-f.com/we2500typ.html#Keysets
http://www.paul-f.com/weAutovon.html