The first pictures (plus the very last one) are from an early (1950?) cal.135 finished in y.gold, with the peculiar fact that it does show a bi-metal and cut-open balance. Something about which a couple of technicians experienced with Zenith declared: "never seen before - only the closed glucydur type".
So then the question arises: would it strange enough be a non-standard modification - why and by whom?
Recently some photo's popped up (on an Italian forum website) and low&behold, here was a similar watch - clearly from the same series - with identical balancewheel ! So at least it is not unique.
In addition, a technical German watchmagazine from 1954 was found which contained a 135-description where this particular lay-out was mentioned in the text and shown with an illustration as well. The movement serial # is somewhat vague but seems to be 3.68m.
After seeing this, it is a reasonable assumption that the first production series were equipped with the cut bi-metallic type before Ephrem Jobin and his team changed over to closed glucydur ones, which are said to have better a-magnetic properties.
best, Ben.