I found the Tektronix programmable plug in's completely by accident. When I acquired my first one, a 7B90P, I thought I was purchasing "just another time base with pushbuttons" for my 7904 mainframe. After receiving it and finding it was not functional, imagine that, I began researching and found that Tektronix had made 2, IEEE488 plug in's for their programmable digitizers. The 7A16P was designed for a 7612D digitizer which had it's own time bases and the 7A90P for the 7912D which had plug in time bases. A few month later, I purchased a 7A16P which was guaranteed and was in like new shape. It could also be used in my 7904 and after using it, I fell in love with it. There's nothing like the touch of a sensitive pushbutton to change ranges.
To make a long story short, the long version will be found in my 7612D web page later, I now have 4 fully operating 7A16P plug in's. The latest 2 acquired with a 7912D digitizer.
What was interesting about the last 2 I bought, one was still in calibration!
The plug in has an interesting hybrid chip which is a bus current switch.
This is what the 2 sides of the plug in looks like.