EARLY 1965 intake manifolds
 

About the first 4 months (Aug. ~ Nov. 1964) of the 1965 GTO build, both the Tri-Power and 4 BBL. intakes had a threaded hole located in the flat boss at the front RH side of intake.

A 1/2” square head steel plug was installed at the factory as this threaded hole was not needed for the ‘65 and newer models.

I’m assuming this was an engineering carry over threaded hole that was used in the previous year, 1964 for its heater hose.

Some of the following photos are poor because they were captured from auctions, but as you can see the dates are Early.

Early ‘65 4BBL. intakes.

The first intake is my Sept. 15, 1964 showing a new square head plug next to the factory plug.

The other 4BBL. intakes are Aug. 31, 1964 and Nov. 27, 1964.

If someone is attempting to have their Early GTO date code matching to an NCRS type level for shows than this steel plug should be there.

Why in 1964 didn’t they just put the heater hose in the hole that has the plug on the right side of the manifold where it is on the big car ?

In 64 they did use that hole in the front right of the manifold. The OP is talking about early 65 applications. Not sure why they changed it for 65.

Here's a picture of our 64 using that front right hole.

I've seen a few of these '65 intakes with the hole for the heater hose nipple. Apparently, engineering made the heater hose routing from the cylinder head late in product design. I simply install a 3/8" NPT pipe plug in that hole. I'm sure some have found an innovative way to utilize it for an added coolant temp sensor or other device.

EARLY 1965 intake manifolds
 

Chris, interesting differences some of these cars had. And at the other end of the 1965 calendar, 1966 parts were showing up early. My '65 4bbl intake 9778816, date G135, has the 1966 PCV System with the 90 degree angle PRESSED IN hose fitting, the nylon hose connector, and the molded vent hose. My '65 has '66 093 heads as the original 77 heads were no longer rebuildable, so I went with the service replacement heads per the PONTIAC CRAFTSMAN SERVICE NEWS, No. 6 - December, 1965.