The Type 81 contains a two-stage amplifier. The letter-series plug-in drives the input stage of the Type 81 which is a high impedance, similar to the input of a 530-, 540-, or 550-series scope. The second stage of the Type 81 is a cathode-follower, which drives the 580-series scope using a low impedance, as is required by 580-series scopes.
The 81 and 81A are significantly different. Type 81 is tube-based and has bandwidth of 33 MHz. Type 81A is solid-state and has bandwidth of 50 MHz.