I think they are electric. The back of the gauge only has 2 wires, and no vacuum nipple. I think it's specific only to S15's too. You can use the holder in S14's though, but you have to cut the top bit of the plastic to fit.
Newbie here on this forum....Have just moved to KL and brought my trusty zed with me. She is a black 94 series 4 Z32 twin turbo. I know a fair bit about these cars as this is my 2nd...