100% late 70s if not 80s. 76s had no legos IIRC, IDK about 77s, but 78s had mustard legos (maybe other wonky shit during that crazy year) but the first fully populated cherry lego boards I saw started in ‘79 and went on from there.
I'd guess 1980...
'77 green 'chicklet' colored caps.
'78 Yellow legos & white 'chicklet' caps mainly.
'79 Yellow legos & tan colored caps mainly.
'80 Red legos & tan colored caps early changing to mainly red legos.