The 81 and 81A are significantly different. Type 81 is tube-based and has a bandwidth of 33 MHz. Type 81A is solid-state and has a bandwidth of 50 MHz.
The Type 81 contains a two-stage amplifier. The letter-series plug-in drives the input stage of the Type 81 which is high impedance (grids of a 6GM8 stage with T-coil peaking) so that the plug-in sees conditions similar to the input of a 530-, 540-, or 550-series scope. The second stage is a 6DJ8 cathode-follower, which drives the 580-series scope's 93 Ω input impedance.
Type 81A has a FET source follower pair followed by cascoded BJT amplifiers.