I believe all of the Heathkit radios were five channel.
Then you would be mistaken. While they started with the GD-47, a 5 channel with the Bonner sticks, they soon revised it with the GD-19 with Kraft sticks and smaller airborne system. These radios were followed by a wide array of systems, from 2 channel to 8 channel systems, some with 2 sticks, others with single sticks. Here is the link to the RC Hall of Fame which has pictures of some of the Heath systems.
Your description of your early system reminded me of the first Heathkit radio that I ever seen. It too was installed in a Taurus, painted all black. The builder of the radio and airplane was a grad student that I met at a on campus flying site (KSU football statium parking lot). Anyway he had decided that the shafts holding the gears in the servos were just not strong enough to do the job, so he drilled out the servo cases and gears with bigger drill bits, then broke off the drill bits to provide the shafts. He did this to all four servos. While that was impressive, the way he mounted the servos was even better. He used a 2 x 4 stud, cut to the length of the radio compartment (at least a foot long), and made cut outs for each servo. Fortunately that day he forgot his transmitter, so I never had to see if it would fly. This guy admitted that building the radio was a struggle, since he was also color blind, which must have made sorting out resistors a lot of fun.