Wow, Amer, nice find and interesting topic, can I say this..
What if they were transitional models before switching to a different case manufacturer...
I bought this Vintage Heuer Autavia siffert with tachy bezel from
: the first owner here in Switzerland ,I was very excited to have
: now 2 siffert second execution Flat V and I took the other Flat
: V to make the comparison and I noticed that the decimal case is
: slightly longer as the tachy siffert and the side finish is also
: different the case opening pushers cat also different and the
: lugs cuting and finish is also different although both are
: serial 142XXX .the tachy siffert is from the first owner and he
: have all the siffert executions including the chronomatic ,mine
: have also the original Bracelet dated 1/69 ,I`m happy to
: discussing this 2 watches with you,here some photos.perhaps it
: was discussed before about those cases difference but I never
: noticed this difference before.