C. P. Clare Series S870
Series S870 is a type of keyboard switch from C. P. Clare. The only known mention is in a C. P. Clare advertisement in Chilton’s Control Equipment Master, 1978, headed “C. P. CLARE KEYBOARDS AND KEYSWITCHES”. The advertisement refers to “Seven quality keyswitch products from which to choose”, these being: S820, S830 and S880 reed switches; S870 “gold-crossbar switch”; S980 “solid-state switch” (foam pad capacitive); and “Slimtac and Softac micromotion pads”. Quite what S870 is exactly is not known, as there appear to be no other references to anywhere. Electro-Mech Components do not possess a catalogue page for it, just as they do not have the S840 page.
Presently there is no known General Instrument or C. P. Clare keyboard type missing an identity for which S870 could be the answer.
Note that Google Books OCR misread “S870” as “5870”, meaning that it won’t be found either in a search for “S870”.