In period mode on the A input, with the level/sens dial pointing straight up (12 o'clock) the display bounces between 99.99999 -9 and 100.00000 -9. Any chance this means something.....useful?
First, the position of the knob isn't calibrated so that doesn't really tell you anything. Second, yes the counter display is rounding down and then rounding up so the counter
thinks that the input is between those values. Again the counter may not be (Probably isn't) absolutely accurate. If you had an input that you could vary the frequency slightly then you would be able to see the ratio of round-ups to round-downs change so the counter can see the difference but I don't think that that change is display is really useful, particularly on a counter that has lot of displayed digits. If you had a high accuracy counter that displayed only a few digits then the change might tell you something but not on a counter such as this one.
"Any chance this means something.....useful?"
Not on a counter with unknown accuracy.
Keep in mind the old high school math lesson that writing down a lot of digits in a number doesn't necessarily mean that that number is accurate to that many places.
A few months ago I picked up a whole pile (
of EIP 548A 12 digit counters with the high stability oscillators in them. It was quite interesting to couple them to each other and to my HP and Racal Dana counters and see how they compared to each other. Most agreed very closely but a few didn't. I was pleasantly surprised to find that about 1/2 of the EIP counters agreed with each other and to my option 004 HP 5343A out to the 11th digit. However that still doesn't mean that they were absolutely correct. No matter what source I used, you could see that each counter had slightly different drift and that the source also drifted over time.
Keep in mind that each counter and the signal source each has its own time base and that all of the time bases are drifting and at different rates. With just one source and one counter you can see the drift but you don't know which one is causing it. When you hook up six or so counters to the same source, you can start to see how each time base is drifting over time.