Best place to look is on the SPID label located on the back of the trunk lid which has all the the RPO codes listed. 4T60E is M13. 4T65E is M15. Also, the 4T65E will have a bigger umbilical cable connector located at the top of the driver's side of the tranny facing outwards towards driver's side versus the 4T60 which is smaller.
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lots of minor things, big stuff is case, valve body, 2nd clutch drum, line pressure is computer controlled, shell (stove pipe) Planet (how it attaches to the shell). All in all nothing is really too different between the 60E & 65E besides design changes on parts.