I can't tell you when the use of the 1965 model year pan started (location oil drain port changed) but it was by August 25, 1964. How do I know, the 48 each HP289 engines made on that date sent to Shelby American for installation into new Cobras received the new version.
What I have not tried to do is look for dates on "1965" oil pans. I have no idea if pans were still being dated by then but pans made 1961-63 were usually dated.
The image below is of the stamped in information in the original oil pan for High Performance 260 engine serial 137 once long ago installed into a new Cobra. Based on my discussion with Mr. Mannel it reads as January 1, 1962 2nd shift (which was a Monday).