Ah, newbie, grasshopper. Knowledge is everything!
When you open the back of the watch you can confirm a 135 in several ways, one is just the design of the movement and the placement of the serial number. Also if you look down into the movement along the outer wall you will see '135' stamped in the main plate. another way is to look at the dial. there are about 6-8 different 135 dials, but mostly all the same except for the Port Royal and the ones with Fabre Leube written on the dial. There is always a small second hand circle at the bottom and the word Chronometre on the dial above Zenith and below the star. These are general rules as I am sure someone will add something I missed. You can see a bunch of the dials by Googling 135 cal Zenith and then click images.
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