News:
I believe there are 2-3 different ones , I know for sure there were at least 5 different gear ratios available Passon or Brewers would know for sure
The '70 and earlier drive pinion has 24 teeth, '71 and later has 25.
I have not been able to find any info indicating that there are different fourth gears other than the coarse or fine spline. Here is a site I found that sells parts and kits. http://www.tbtrans.com/a833_4_speed_transmission_pa.htmAnyway, the trans is # matching to my '70 A66 Challenger so it should be fine spline unless someone changed it.
Mine matches also and it's a 23 spline. I just dealt with Brewers and they had a used 1-2 shift drum I needed. Maybe they have what you need also.