Peter,
You can choose when you take the FAA ATP exam whether its going to be on Part 135 (Air Taxi/Charter) or 121 (scheduled Airline) Regs.
If I remember rightly, 135 flight planning was for the B1900 and the 121 was on the 737/727.
The 135 regs are more akin to the standard 91 regs wheras the 121 regs get more detailed and specific.
Whichever one you choose to do, there'll be absolutely no indication on you certificate about whether you did 135 or 121 q's.