Jim - In '68 the billing docs were different than the '69 and up invoices you have there.
For example, Box 50 might have a 3. Options for box 50:
1 = Power brakes
2 = Power steering
4 = Tilt wheel
In this case, the 3 would indicate power steering and power brakes were on that particular car.
Good link here to '68 PHS docs.
http://collectorcarpricetracker.com/.../290634088535/
As for the trans option, M20 would be the standard 4 sp transmission. If the Performance Axle was selected, 3.90 or 4.33 the M21 was mandatory. 3.55 gear is standard without A/C for both auto and stick cars.
I hope this helps...