Several Tektronix products use miniature coaxial BSM connectors – also sometimes called MB connectors – e.g. the 7T11, FG501, 11B2, 3S2, S-50. S-51, S-52, and 067-0587-01. These look like minature BNC connectors.
There are two-lug variants and three-lug variants. Tektronix used the two-lug variant. BSM connectors are much less common than BNC connectors and are hard to find on the surplus market.
The Tektronix 012-0127-00 adapter converts from BSM to BNC.