If you are interested, you can discover the axle ratio for the '64 or '65 A body by checking the Axle Charts. One Chart shows the Standard Release ratios with Standard Brake linings, the other shows the Standard Release ratios with Metallic Brake option. In both cases, the available Special Order ratios are also found.
The ratios (standard and special order) depended on car model, engine choice, trans choice, carburetion, AC or not, Trailering Option, and with or without Safe-T-Track. Of course, the GTO always got the 389.
If you didn't specify a Special Order ratio, you got the Standard Release ratio depending on the combination of the above choices.
If you specified a Special Order ratio, the build required Approval and it was only approved if the specified ratio was permissible according to the Axle Charts that, again, depended on the combination of the above choices.
You couldn't, for example, specify a 2.56 axle for a '64 or '65 GTO regardless of any other option selected. The 2.56 axle was offered for the A body, but you could not order it in a GTO, it would not be approved.
In '64, at the start of the year, you would get the 3.55 ratio as the standard release axle in a GTO with Manual Trans, Tripower, no AC or trailering option, Standard or Metallic Brake linings, and with or without Safe-T-Track.
In January '64, PMD announced a change for this combination, the 3.23 axle was now the standard release. You could still get the 3.55 axle but it was now a Special Order choice for this combination. It was permissible, so it would be approved.
The Approval is noted on the '64 Manifest docs available from PHS for any GTO that was ordered and built with a Special Order ratio.
Curiously, for '65, this combination reverted to the 3.55 as the Standard Release ratio.
In '64, PMD had a 3.90 axle with Standard Brakes for a specific combination in the A body. But you absolutely, positively could not specify it for any '64 GTO regardless of the above option combinations.
You could however special order a 3.90 axle for the GTO. But it required mandatory extra cost Metallic Brake option, HD Radiator, HD Fan, and Safe-T-Track options. You could NOT get it with AC. With Man Trans (3 or 4 spd), you could get it with the base 4 bbl or optional Tripower. With auto trans, you could only get it with the Tripower (and an extra cost Speedo Adapter for the trans was also required). Late year (nominally May '64), you could order the newly released M21 close ratio 4 spd but the 3.90 was mandatory along with the other extra cost options required with the 3.90 and it was only offered with Tripower.
I've never come across a '64 build that had the wrong axle installed by error but I'm sure it was possible. I have records for plenty of '64 builds that were ordered with a Special Order ratio and plenty more that were built with the Standard Release axle for the particular combination. Always conforming with the Axle Charts.
The OP indicates the '65 GTO build in question included the 4 bbl and 4 spd without AC. He didn't mention other options, so it isn't possible for me to know whether the 3.23 axle was included as the Standard Release for that build or by Special Order. But it was certainly common for a '64 or '65 GTO to get the 3.23 axle when NOT equipped with AC and the indicated combination.
I believe he was confused by the fact that the '65 GTO with AC and Tripower commonly got the 3.23 as the standard release (unless some other uncommon trailering option was included) while it would have gotten the 3.55 without AC. The 4 bbl GTO with AC combination often got the 3.08 in lieu of the 3.23 without AC.
Consult the Axle Charts for complete specific combinations.
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