Hi Paul, am I looking in the wrong place, or is the Days Limit somehow missing from my control panel?

You seem to be missing several options, all of which are standard in all 4.2.x versions.

Are you sure you are runing 4.2 ?
If so I suggest you make sure you have all the latest 4.2.x files uploaded, and re-run the upgrader (and remember to remove the install folder again after).

Any idea why certain public threads that were created since installation of this add-on would not show any visitor instances? The specifics are that I've had this mod installed and working since 2014 and am now finding that any members who have posted to threads prior to September 23, 2015 are not shown within the "Members who have read this thread" display. Is there something that could account for no greater than a 6-month retention period? Or might something else be wrong? No other issues have been noted. The directive at Options > Thread Display Options (showthread) > Who Read - Days Limit is, and has always been set to zero (0).


EDIT:
On the help item for Who Read - Days Limit, it says:
Maybe that's the problem. The auto clean-up is biffing the old records. But would it be after 6 months?

What value do you have it set to ?

As I mentioned in my first post, it has always been set to zero (0), as it still is.