Information needed about my Seamaster Chronograph cal. 321

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Hello all - I'm in need of some help. Several years ago I purchased this Seamaster Chronograph. At the time I didn't give it much thought, the price was very reasonable so I bought it. I had my watcher (who was an Omega trained true watchmaker) service it. When I got it back I was told it has a 321 movement with a ref# of 20525760. I put it away in the safe not thinking more about it. Sadly the watchmaker passed away so I'm in need of some expertise. It's been a while since I've posted anything on the OF but I thought maybe some of you could help me get a little more info on this watch. I cannot find any with a 205 ref#. Any help would be greatly appreciated.
 
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ref# of 20525760
This is not the ref# but the serial#. Think it's possibly a 145.005 67.
Edited:
 
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YY77 - Thank you for you input. I found and added a photo of the movement and see the 205# I noted is the serial#.
 
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Great piece, just a stunning dial. Wear it in good health
 
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From my knowledge the serial number has an error of approximative 1-2 years..so you can not tell 100% that the watch is from 1963.
 
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20.52m on a 321 movement would indeed usually indicate 1963 or 1964 manufacture. The piece of the puzzle you haven't provided is what is stamped on the inner side of the caseback. Usually there will be case number and other info. The movement serial isn't enough to identify the exact model. One other thing of note is the OXG mark suggesting it was sold in the USA, were there US locally produced gold chrono cases I wonder? If so that might make ID a bit trickier.
 
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I’m concerned that outside of the caseback is unmarked. Everything I’ve recently seen online has “Waterproof” or other markings. As for the inner side, when I bought it my watchmaker had the caseback off and took a quick picture but the photo is blurry…sadly he’s passed. I should to find a qualified watchmaker and open it up again. More pieces to this puzzle though - thanks paddlers!
 
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I’m concerned that outside of the caseback is unmarked. Everything I’ve recently seen online has “Waterproof” or other markings. As for the inner side, when I bought it my watchmaker had the caseback off and took a quick picture but the photo is blurry…sadly he’s passed. I should to find a qualified watchmaker and open it up again. More pieces to this puzzle though - thanks paddlers!
Hi there! I have an omega seamaster ref: 105.001-62 with a 20.52 serial and it also hasn’t got any markings (waterproof) etc. on the outside of the caseback which was concerning to me also but nice to see someone else has it too!