Hi, this probably is beating a dead horse, but I finally acquired a datasheet for the 6RS20SP4B4, used in the LT33 teletype modifications. I have scanned it and made it available at http://so-much-stuff.com/pdp8/pdf/6rs20sp.pdf (The data sheet actually describes the 6RS20SP and 6RS5SP families.) Contrary to DEC documentation, it is a thyrector (not a thyractor, which would have an inductor in it). Vince