It is indeed a 5NW code, stamped in plain view on the data plate.
For the information of all, I have primarily been using the GTO Association of America's Pontiac GTO/GT-37 Illustrated Identification guide. I bought this guide just a few months ago at the GTO Nationals in Loveland, CO.
I have already found a couple of pieces of information that are inaccurate, and I am not writing this to slam the publication, but I am writing this so I can inform yourselves and inform the GTOAA as well (I will show them this thread; this way, I only have to write it once).
The first piece of seemingly inaccurate information is found on page 102 at the bottom, which discusses "How To Read Production Code Numbers. This information, coincidentally, is found on the same page 102 where the 1965 transmissions are discussed.
Anyway, without typing the whole darn thing, it basically says this: P2E17 A.
P means passenger, 2 is model year (1972), E is month (May), 17 is day, and "A" is a suffix code for the ratios. From what I have found, this information is entirely true for LATER year GTOs, but not for the 1965 in question. The P0305 actually means third month fifth day, as evidenced by this commentary at Medatronics Corporation's 5 Speed Home page!
Serial numbers for Muncie 4 speeds always begin with the letter "P". P stands for Muncie Plant. Not for passenger car as some self proclaimed specialists may think. The letter M was used to ID the Muncie plant for the Muncie 3 speed, so P was the next logical letter. The letter O would add confusion since it resembles zero. The serial number is a date code the transmission was built for a particular year. Serial numbers from 1963 to 1966 included only the month and day. P0101 would indicate January 1st. From 1967 to 1968 the serial number got a year designator and a letter designator for the month such as P8A01, meaning January 1st 1968. One important point is that if you have a Muncie dated with a December build date it was actually built the prior year. An example would be the date code P8T13. This is for a 1968 production car. The T stands for December and 13 is the day. To confirm this simply look at the VIN number. It will usually begin with a 18S101350 or a 28N12950. This means the Muncie was assembled December 13, 1967 for the 1968 model year. The VIN number will usually be a low number. 1969 to 1974 Muncies got a ratio designator at the end of the serial number. An example would be P4D23B. This equates to April 23, 1974, M21 ratio.
Imagine my horror just about an hour ago when I discovered the date code makes sense on this 1965 GTO's Muncie. Today, I was looking at the car and found the VIN number perfect, the two production numbers perfect, yet the date code looked weird. Anyway, I spent a gross amount of time on the transmission codes today and now find out that the GTO publication needs to clarify their explanation.
Trust me, please. This is certainly not a rant. It is only to provide information. It just goes to show that not everything you read (or sometimes see) is true.
I realize that the 5speeds.com site may also be inaccurate, but the "0305" just makes too much sense here.
On another note, the GTO I.D. book lists a R.H. exhaust manifold as part number 9779325. And although I am not nearly as confident as I am with the transmission deal, all literature I can find shows that part number indeed belongs to a 1968 GTO. Now, you may say that I should go verify that on my own 1968 GTO, but mine is an H.O. car and as a result uses a different part number altogether.
At any rate, I want to thank all of you for your responses thus far. Please keep them coming. I have always viewed forums as a wonderful place to learn something new, and this forum is no exception.