I may have spoken too soon. I started to think more about the DC509/DP501 relationship and looked into a couple of other TM50x power modules I have here and was quite surprised to find that the "prescale" command is carried through the backplanes in the group of wild card busses Tek uses for various options. This allows the DP501 to directly tell the DC509 that it is in the prescale mode so that the counter can do the correct math for the display. And I have never had the opportunity to use this combination of modules in any other of my TM50x power modules to have discovered this . My bad.
What I did find out yesterday was that, for some reason, this TM502 I use seems to be a special revision apparently for dedicated use with some foreign (non-Tek) modules that required a special configuration of these busses.(backplane contacts A14 & B14 through A28 & B28). I am somewhat aware that there were some of these produced for certain applications.
I believe that use of some earlier version counter modules may either still require backplane modifications or forcing the user to multiply the reading by 16 to get the correct result.
Sorry for the confusion! I still learn something new each day.