More like 1967 for that batch of Carreras Bryce, though dating pretty much any Heuer is an imprecise science and I'd always allow at least 6 months either way.

It's all done from precedent, whether that be sales documentation, engravings, known model introductions or whatever. Note that we don't have the luxury of archives that Omega, Rolex, PP etc collectors do but a few years of spadework have produced a few fairly extensive spreadsheets and databases amongst collectors. The sample size doing it like this will only ever be a fraction of production though, so the dates shouldn't be taken as gospel - they are effectively our (collective) best guess with the info to hand.