Anyone know info or a good link on how they achieved polyphony for Synthesisers in the 1970s? Specifically difference in approach between the 'Polymoog' and the Yamaha GX-1?

The only polyphonic synth that I'm reasonably familiar with -- being a Hammond organ fan -- is the Novachord from 1938, which was a behemoth:

QUOTE: Containing 163 vacuum tubes and over 1,000 custom capacitors, the Novachord weighed nearly 500 pounds and was roughly the size of two spinet pianos. The divide-down oscillator architecture, based on vacuum-tube monostable circuits, permitted all 72 notes to be played polyphonically by deriving several octaves of notes from twelve top-octave oscillators. A similar design was adopted in polysynthesizers released more than 30 years later by Robert Moog and ARP.

The Novachord may have used "divide-down" (like the Polymoog), may have been the first divide-down machine!

It does use divide down, as stated above. Has 12 master oscillators.

Funny didn't see "divide-down" when originally glancing at quote, stopped at "163 tubes" and guessed that's what Hammond was up to...

I can see why Bob Moog's 'Polymoog'(1975) imitated the Novachord 35 yrs later: frequency stability!

From what I can gather of other "true" analogue poly synths that use voltage-controlled-oscillators (VCOs), they may only have, say, 8-note polyphony from, say, eight monophonic type VCO modules that are "agile" (can be gated on & assigned a pitch voltage relating to note being played - up to (8) "simultaneously" (actually millisecond priority juggling) as with famous Yamaha CS-80 (1977)? (anyway that's how I'd envision it, not finding a tutorial.)

Both Moog & CS-80 being pre-microprocessor and thus their control system implemented with boards of hardwired chips.

The Yamaha and other true VCO synths - but not the Polymoog - being bedeviled by temperature induced frequency pitch-tracking drift which inspired the next great idea: the Digital-Controlled-Oscillator (DCO) where (no doubt?) a divide-down frequency reference was included, and provided a correct syncing that was also assigned to the relevant VCO(s). Not sure what year DCO appeared?

Wiki has a good page on DCOs, they say Roland may have been the first to use them ~1980. Then they say 'may as well go all-digital using things like DSP chips'.

Only problem with that was the contemporary backlash from whining musicians who claim old analogue synths sounded better, thus the return of the (DCO)-VCO in some present products: there being an 'arms-race' amongst the crowded Japanese brands: Yamaha, Roland, Korg, Casio, Kawai, Suzuki... Kurtzweil (Korea)