This is from the GTO forum: "
GTOs had High Torque starters with special coils and armature. From 65 to 69, the HO and RA 1108353 starters were a bit more Heavy Duty than the 1107355 used in the base GTOs. The solenoid was also a heavy duty version. A quick way to determine high or low torque starters is the HT starters had larger coil windings and case. This required a copper spacer bar between the windings and the solenoid motor terminal." Some full size cars had heavier duty starters as well, I've gone by the presence of the copper spacer bar mentioned.
https://www.gtoforum.com/threads/sta...ration.141542/