Comments:
It stands for 40-T, one of Zenith's Observatory Chronometer movements...
It could find out the following:
This message has been edited by Ornatus-Mundi on 2014-10-29 06:39:09
The Cal. 40-T is a 12" hand wound movement (thanks blomman for the pointer!) beating with 18.000 bph, produced between 1947 and 1963 in a quantity of 162.000 pieces. There were standard and chronometer-certified movements. It is Zenith's second-famous chronometer movement (after the slightly larger Cal. 135).
Here are a few images from a fellow Purist (of a non-chronometer 40T):
The particular watch in question is identical to one owned by famous watch journalist and historian Christian Pfeiffer-Belli. Assuming that all is still original and no parts (movement or dial) have been exchanged, chances are high that you own a true chronometer piece.
Hope this helps!
Best,
Magnus
A comparative pic between a chronometer-certified and a standard T-40...
which I found on the web:
(left, non-chronometer; right, chronometer version)
(left, non-chronometer; right, chronometer version)
Note the difference in finishing as well as in the balance wheel and the hairspring stud. Furthermore, the chronometer version is individually numbered also on the barrel bridge.
Hope this helps!
Cheers,
Magnus