In a message dated 1/12/04 2:16:31 PM, vanagon@VOLKSWAGEN.ORG writes:

<< Probably to prevent air
locks.  If there is a pocket of air next to the thermostat it will take a
lot longer to open. >>

Brownian Motion seems to be taking this thread to a conclusion.

If there is a pocket of air next to a thermostat, the coolant system has not
been filled properly.  The small holes seen in the various control elements of
a hydronic system are there to help in venting the air from the system when
it is being filled.  Once it has been filled, these vent holes have no effect
in regulating the system.

Filling a Vanagon cooling system in some manner even remotely akin to what is
prescribed by the shop manual, does not require drilling a hole in the
thermostat.  If a vent hole were necessary, I assure you one would already be in the
thermostat plate whenever the VW parts man handed it over the counter.

Regarding the subject line:
As an expediency while renewing the many bits and pieces of a Vanagon cooling
system, I installed an aftermarket thermostat.  Although it was marked with
the correct temperature, the van ran too cool and within a week I replaced it
with the original thermostat which had seen 127 Kmi. of service.  Judging by
that nuisance, I have to say that all thermostats are not equal.

George