OK - that's one! Any more
Some of the early SWR meters, e.g. Heathkit, had a small slotted line that directly sampled the forward and reflected fields. I had a pickup loop, taped to my parallel feedline, with two attached flashlight bulbs that glowed with forward power and reflected power. Does that count?
Sorry, no. That's a directional coupler. My slotted line device demanded that the operator physically move a RF probe back and forth inside a essentially a coax with a slot cut lengthwise. So, you would move the equivalent of a quarter wave to get the max and min RF voltage on the center conductor. THen do the division of Vmax / Vmin = SWR. Can you imaging adjusting your antenna tuner doing all that? You had it easy, even with the old HK.
I forgot something. You can measure the wavelength with the slotted line also. Probably won't get you the accuracy of a HK GR64 even.