This is a special design of jack made by Switchcraft that is used by Buchla (and many of their clones) to carry audio signals. They are 3.5mm in diameter, but differ slightly physically from a common 3.5 mm jack. 1/8” plugs would be loose in when plugged into a Tini-Jax jack; a Tini-Jax plug might not fit into or might even damage a 1/8” jack.
