I was advised to read up on the Propeller Owners Manual and look at the Maintenance Practices section.
It turns out the explanation is there, along with a Vol. 11 of Manual 202A
It is a function of the grease used.
Aeroshell 6 is the most common all purpose grease, and apparently used in all new production propellers since 1989 except for a few exceptions, notably the Piaggio P180. No limitations noted.
. . .but. . .
Aeroshell 5 has good high temp. properties and very little oil separation or leakage. It has the -40C limit. This product will limit the operator to flights warmer than -40C.
It is possible Aeroshell 5 was used (or mixed with other grease), on the aircraft at some time in the past, so the placard makes some sense.
Now, all I need to do is find the grease log entry and see what was used on the last overhaul.