I have an old moldy copy of a book on circuit training written by the original developers of the technique, R.E. Morgan and G.T. Adamson from Leeds University. It smells weird and the pages are brittle, but it is very interesting.
There were no machines used in this gym, it was all full body gymnastic and strength work that would be pretty hard for a lot of relatively fit folks to complete. I'm going to list some of the exercises and try and provide a description that is relatively accurate so you can see what I mean.
Rope Swings: So, picture this, you have two ropes about arm length apart. You must grab a hold of both, take two steps back and then heave yourself forward with enough force that your feet touch a horizontal beam that is approximately 8-10 in the air.
Arm jumps: They utilized a ladder placed horizontally at "jump height. So, picture doing this on the monkey bars. Hold on to the outside support beams and by pulling up explosively, use your arms to "jump" your way down the bar.
Arm walk on parallel bars: So, just like it sounds, press yourself up in a support position on the parallel bars and walk though it on your hands (you are not in a handstand position, you are hanging between the bars).
Barbell Swings: This is very similar to a clean grip hang power snatch, except that you swing the bar overhead like a kettlebell swing.
Barbell Curls: This is the same as the barbell swing, but with an underhand grip. Very much like a power curl.
Wheelbarrow lift: They have it set up to look like a partial deadlift, but the action is the same as lifting a heavy wheelbarrow.
Dumbbell Jumps: Holding a dumbell in each hand, place both feet on either side of a 12" bench. Jump onto the bench by jumping up and bringing your feet together.
Squat Jumps: This is sort of a jumping B squat. One foot is slightly in front of the other such that when you land, your back foot is on the toe as in a lunge. Alternate feet with each jump.
Dumbbell squats: Holding dumbbells in both hands and with heels supported on board, squat to the ground.
Jump and heave: This looks like a jumping pullup, but chest to bar.
Jump and press: Using the parallel bars, jump up to acquire a support position. Dip slightly and extend the elbows explosively as you "jump" down the bars. This seems to be an explosive or plyometric dip.
Squat and press with bench: So, they would hook a long bench (10-12 feet) to the wall. Then, holding onto the end of the bench, they would squat to the floor and then come all the way up, pressing the bench overhead until they were on their toes. Similar to a thruster, but stabilized on one end.
These don't require as much explanation:
Rope ladder climb
Burpee: no pushup, no jump
Barbell squats: Butt to ankles.
Bench Stepping: Up and down 2-3 rows up bleachers
Parallel bar dips
Now, I can't vouch for the safety of some of these as I've never used them before, but its interesting to see how some of these have clearly evolved over the years and have been integrated into other programs. Some have been dropped altogether. These circuits were performed with 40% 1RM training loads and were not done for time, just good form. About 20-30 seconds of rest was allowed between exercises as individuals moved from one to the next.
Circuit Training, R.E. Morgan and G.T. Adamson
The Camelot Press, Ltd., Great Britain 1959