The 7A18 is a very common dual trace vertical amplifier plugin for the 7000 series mainframes. Its 75 MHz bandwidth can only be attained in an appropriate mainframe.
This one equipped with option 06, which includes dc offset circuits that provide up to 200 division of baseline offset. Internally, up to 1 V positive or negative dc offset is added directly to the input of each amplifier. This results in an effective practical offset range of maximally 400 V with a 10X probe. Theoretically, the range goes up to 10000 V. However, this is probably be out of the maximum input rating the probe and/or amplifier.
The variable offset knob replaces the indentify button of the optionless model.
Front of the 7A18 option 06. Note the extra DC OFFSET position of the input coupling switch. The identify inner shaft button has been replaced with the variable offset control. Compare with the normal version.
Schematic of the input amplifier without option. A single-ended jfet source follower drives one side of a cascoded bipolar differential pair chain. The diffpair is balanced by applying the appropriate dc voltage to the unused input.