G
The Tektronix Type G is a plug-in differential amplifier for 500-series scopes.
Unlike the Type D, the Type G is optimized for RF, achieving only a 100 to 1 common-mode rejection ratio but operating all the way to 20MHz. Its maximum sensitivity is 50mV/div. It has seven tubes.
Its name changed twice. Type 53G was introduced in March 1955 along with the 541/545, but it was only spec'ed to 10MHz in a 535, and its performance in a fast mainframe was not documented. When the mainframe cabinet styles were revamped in August 1956, the Types 53A, 53B, 53C and 53G were all upgraded (sped up) and renamed; the 53G became Type 53/54G. It was the only early plugin that did not go through the awkward "53x/54x" phase. In 1959 it became Type G, with no further change in design.