An 868 is a 4 pin photo tube. This tube is used to detect differences in light. Can be used to detect something crossing a light beam. The 868 was also used to detect the sound track on film.

A 551 is same as 35 tube. Looks very similar to 24A but is remote cutoff. It takes a greater negative grid #1 voltage before the tube stops operating.

Norm

:I found in my stock a RCA radiotron 868 tube (4 pins) and a Rogers A/C 551 (look like 24A with globe shape).
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:I don't know what they are.