This is a temperature sender with a red top for a Mini 1964 to 1969, MGB 1968 to 1976 (RD from (c)138401 to 393763, GT from (c)139472 on), MGC and TR4 and TR4A

Most minis with dual gauges up to 1969 including Cooper S Mk2 that had oil/temp gauges fitted used this type and all when fitted with a voltage stabiliser.

Minis before Sept 1964 except some super deluxe and pre Sept 1964 Cooper without voltage stabiliser used GTR102 with green identification nose.

After 1969 use GTR101.