There is not much online about it.
Tektronix used and described the procedure in a couple of their service manuals where it was used for instrument calibration.
I suspect there is a detailed article about it in one of the TekScope publications but I have never seen an index of them to know where to look. The 7A13 was first produced in 1969 so that would be the time period to search.
There were earlier differential comparators as well and the operating manuals for them may describe how they can be used to make slide-back measurements.
The term also shows up in some old NIST publications.
Update: I did a search and found articles in November 1972 TekScope and January/Feburary 1973 TekScope but they are part 2 and 3 so there is an earlier one which I do not have. They do not discuss the 7A13 differential comparator specifically but part 2 mentions slide-back measurement:
Let's see how we go about making a differential comparator measurement. First, we establish a reference position on our display by grounding both inputs. Then selecting the appropriate input (positive voltage source to the + INPUT, negative voltage source to the - INPUT), we switch the other input to the comparison voltage (Vc). Next, the comparison voltage is adjusted until the trace "slides back" to the reference position. What have we accomplished? Using the "difference" principle we have introduced a "common-mode" condition in the form of the comparison voltage; that is, the comparison input voltage now equals the signal input. We see that we now have the ability to measure any potential whether it be DC, complex in nature, or a combination of both - such as a complex wave superimposed on a DC potential. Thus, we have an extremely versatile measuring tool.
The reason you would do this instead of reading the voltage directly off of the oscilloscope is increased precision and accuracy.