A little while ago, I made a thread on waterproof 13ZN references (https://omegaforums.net/threads/waterproof-longines-13zn-references.120287/). One reference that I did not include was 3870. This reference is relatively unusual in that it can be found with two different styles of chronograph pushers: rectangular or round. I have not seen a large number of examples with round pushers, but all seem to be from order 21'153, marked "LXW" on the balance cock, and invoiced to Longines-Wittnauer (USA) between 1941 and 1944. The case-backs of the examples with round pushers seem to be the same as those with rectangular pushers, neither have gaskets. Below are some examples for reference. Source: Chrono24 Source: https://www.carsandwatches.com/watches/rare-longines-13zn-waterproof-with-black-dial View attachment 1646555 Source: https://secondtimearoundwatchco.com...ister-chronograph-stainless-steel-watch-14239 Note: This example has a refinished dial.