B
The Tektronix Type B is a plug-in for 500-series scopes.
It is identical to Type A, with the addition of a 10x-gain AC amplifier which can be switched in to extend the vertical sensitivity from 50mV/div to 5mV/div.
Like Type A, the Type B has its roots in the 1954 introduction of the 531 and 535, as the Type 53B, with 10x-AC bandwidth of 2Hz to 9MHz. In 1956 it was replaced by the Type 53/54B with DC-20MHz or 2Hz-12MHz bandwidth in the 541 and 545. In 1959 the Type 53/54B was renamed Type B, and it was available through 1970, a seventeen-year run in all.