It depends which part of the 60's!
Early 60's Lowreys were valve generated - no mechanicals - and from about 63/64 they switched over to solid state electronics. Subtractive synthesis is the technique used. Take a waveform rich in harmonics and then filter out what you don't want.
Very different sound to the Hammonds of the day, very pure flutes, but adding the strings and reeds made them really 'fizzy' through a leslie. After about 1975 there was a marked change in the sound that Lowrey
made, as they changed 'direction' towards orchestral sounds, and I'd
regard the TLO Holiday and GAK series as being the last of the 'old school'
Lowreys. TLO's, GAK's and GAKH's etc can often be picked up for 99p / $1 on eBay these days. Very few of the spinets fetch much more than that, so it can be worth a go. Good valve models include the Heritage and Brentwood.