1A6
The Tektronix Type 1A6 is a low-cost differential plug-in for 500-series scopes.
A replacement for Type D, it senses down to 1 mV/div at 2 MHz bandwidth, with 10,000-to-1 common-mode rejection.
Type 1A6 was introduced in 1967 and dropped after 1973. Not as many units were sold as the more sensitive Type 1A7 or the faster Type 1A5.