"Thyractors" on the DEC LT33 modified ASR33 Teletype

[Philippe Sonnet]

Hi folks,

I am currently modifying my Teletype ASR33 into a LT33 according to the DEC 
specifications to work with my PDP-8/L. Does someone know what is a 
Thyractor (GE 6RS20SP4B4) ? Some kind of voltage clamping device ?.
How could I make one, using, for instance a pair of thyristors or a pair of 
zenner diodes ?

Many thanks,.

Philippe Sonnet