Exhaust Relief valve

I see only one possibility...
It may be an exhaust powered lift system remain.

I remember that some small Farmall & McCormick
tractors had exhaust powered lift system instead
of hydraulic lift.

If it is an exhaust powered lift the remains of
the system may still on the tractor such as
pipes to the rear or special lift cylinder.

HTH

Joe Freguson

I added the link to your photo here in order to be seen by others. I spent 50 years around all kinds
of tractor and machinery but can't figure what it is.

Here is the link to the picture
hope it works.

Link

Joe Ferguson

Better to put the real picture here

Bye !

I take it all back - it isn't what I thought it was at all.

Stumped, clueless, ignorant, lost, dumbfounded.

• Hollywood Wolf Whistle

This hole is a vacuum port used to put a vacuum
gauge for testing purposes. Some tractors with
cab many years ago (pre-1960) used vacuum operated winshield wipers (as the cars of this era)
And some farmers used that port to operate milking machines (my father did) during electric power failures.

If there is a valve it must be closed all the time
when using tractor, if there is a hole it must be plugged otherwise it will cause major vaccuum loss and engine malfunction.

Hope it helps....

Joe Ferguson