The Autavia Chronomatic has some unique differences to the later model after 1970.
First obviously we have Chronomatic on top of the Heuer logo, the typo of AUTAVIA is unique, the dial color on the Siffert is a more colder white tone, the case has slightly smaler lughorns, the chronograph pushers are less embeded and stand out more prominent hence less protected or integrated than on the later versions.
On my Siffert shown in my book on page 49 the back case is marked with a small C and numbers, which was done by the Swiss holesaler in 1969 and proofs that the watch was sold new in Switzerland.
The hole case looks a fraction thinner than the 1970 version.
On the Carrera Chronomatic the typeface of Carrera is different with a more pointed A, clearly visible on page 144.
The early caliber 11 marks look slightly different C of CAL less rounded and A more pointed, further some small drillholes on the plate.
Heuer produced between 200 and 1200 watches per modelline and year, production of the Chronomatic was started autumn 1969 and lasted around 6 month.Now we could start to guess ?

Cheers

Arno
www.heuerchronographs.com