Posted for géraldine berger-yvon cournoyer <geryvon@videotron.ca>:

Allo Perry,
My engine is a 1986 13B and The Hole you are pointing to is where Mazda
screws in their coolant temperature sensor. On my engine that hole is
located just below and under the oil filter pad and just above the hot
coolant heater outlet, on the rear housing.. If you have a Mazda coolant
temp sensor it will fit just right in  and plug that hole up very nicely.
Some time ago I removed the Mazda sensor which was unused and rethreaded
that hole for a standard coolant temp sensor. And it works vey well.
Yvon.