Most of those monochrome units were not exotic. Polar Mohr for example used standard N.T.S.C. monochrome. Thermotron and others simply use 3 wires plus common ground at the N.T.S.C. / C.G.A. signalling rate. you have V-Sync, H-Sync and Video. Some are even simpler, Composite Sync on one wire and Video on the other and a common return. Some of the color stuff had a weird Green+Sync and it was designed so the Green+Sync could drive a normal N.T.S.C. monochrome monitor if the video card was jumper selected to act as such. Many of the old separate sync monitors also included a 'phase adjustment' for horizontal centering to correct for horizontal sync timing since some H-Sync video card outputs looked more like a squarewave.