Here is a brief overview of Longines' waterproof, 13ZN references. I might be forgetting a couple but we can add them later. Waterproof 13ZNs are among the most valuable, vintage Longines. As such, one must exercise extreme caution when assessing authenticity and originality. This is especially true for references 4270 and 4974 where fake cases are known to exist. This thread is not intended to serve as a guide for identifying fakes, it is simply a summary. The first, as far as I know, is reference 4270. The back is pressure-fit and the pushers have a unique design (patented in 1938). The diameter is around 37.5 mm. Source: http://www.db1983.com/our-archive/longines-13zn-mushroom-pusher-1939 Next is reference 4813. Again, the back is pressure-fit but the pushers are now in a more familiar style. The diameter is around 35 mm. Source: https://www.carsandwatches.com/watches Next is reference 4974. The back is now screwed to the mid-case and the pushers remain in the familiar style. The diameter is around 37.5 mm. Source: https://www.phillips.com/detail/longines/CH080319/25 Next is reference 5415. The back is pressure-fit and the pusher remain the same. The diameter is around 37.5 mm. This reference also comes in gold with a screwed-back. Source: https://omegaforums.net/threads/1948-13zn-longines-chronograph-seeking-info.68950/#post-867928 Source: https://watchnet.co.jp/en/ Possibly the final, waterproof reference is 5699. The back is screwed to the mid-case and the pushers remain the same. The diameter is around 39.5 mm. This reference is notable for having a heavily modified movement to accommodate a central, chronograph minute hand and a 12 hour counter at the 3 o'clock position. Source: https://www.monacolegendauctions.com/auction/exclusive-timepieces-15/lot-51 An oddity that I will include is reference 4270. I only know of one that has surfaced and I have not seen an extract from Longines, confirming its legitimacy. The case is allegedly 18K gold. If I recall correctly, the back is pressure fit. Source: https://www.instagram.com/p/pjxk-1i5gF/ Hopefully, this was of interest to some. Edit: I added photos of the inside of case-backs where possible. Note that the back of the ref. 5699 has a separate piece, that normally sits around the movement, sitting inside it in the photograph.