Most visitors online was 1660 , on 12 Dec 2020
Burt,I have been taking the easy route sitting and waiting for someone to answer this question. Well since no one wants to take a shot maybe I have a plausable answer. In checking the Hamilton Ledgers on your chronometers mentioned as well as my own could it be be only those Model 21's which were purchased by the U.S. Government and sent for testing at the Naval Observatory have the N and 4 digit number printed on the dial? I referenced a excellent article in the NAWCC Bulletin No.356 June of 2005. This was written by William O.Bennett in 1977(prior to his death in 1994) who was employed by Hamilton in the development of the Model 21. Preformance specifications established by the Observatory required extensive testing which took 30 days to perform.In the artical is pictured a Marine Chronometer Performance Test card which clearly shows that only a 4 digit number listed in the upper right hand corner of the document. Chronometer 2E6125 with the N dial was finished and sold on 9-7-44 to 64-1377 which is preported to be a code number for the U.S.Government. My instrument is 2E5968 with the N dial finished and sold on 9-7-44 to 64-1377.When I checked on the 2eE1189 chronometer it was finished and sold on 11-26-45 and sold to a John B.Machado # 36923. Ok, perhaps this isn't a sufficient number of examples to form a conclusion but maybe it's a good start. The ledgers only list Hamilton's 2EXXX number so to continue this research individuals who own these chronometers could look up and report what they discover. Maybe I'm all wet with my theory but at least I tried.
In 1888 the United States Coast and geographic Survey was a "civilian" organization under the United States Department of Commerce. This organization founded in 1807 ,known by 3 others names during its tenure, was responsible for various responsibilities including "mapping and charting" for our government until being closed in 1970.(F.W.I.W. ) How they, the United States Government, did purchases of chronometers in the 1940'S I cannot definitely say. The Hamilton ledgers are full of sales information to the 64-XXXX designations. This I believe has been accepted by the collecting community as "Government military contracts" without specific information "to whom" in the government it was ordered or received . This specific part of the puzzle is still waiting to be discovered.
I can say with some certainty that in earlier times (WWI and earlier) the United States Naval Observatory made all purchases of Chronometer instruments of all types for the United States Government military services. I have viewed about a dozen of the actual chronometer records which were keep at the N.O. and all are specific that the N.O. made the purchases directly from chronometer manufactures and have other specific information as to dates accepted/purchased and manufactured , price paid and trial numbers. Of course they also contain the complete record of chronometer service. I have viewed a single Negus instrument records which indicated it was purchased by the N.O. in 1868 for $400.00 and after some military service was sold to the "Coast Survey" in March 1888 for $266.25. While this is a single example it's possible that sale indicates either that organization was not a military unit per-say or that perhaps some non-military units were not under the observatory purchase/ maintenance program? That of course brings up the question of then how were those specific chronometers serviced, tested and recycled? If I had to take a guess I would conclude nothing changed in the purchase process during WWII and all chronometers were purchased by and went directly to the observatory for dissemination to the various military units for service.
Just a side note of interest during the 1940's, no doubt because of electronic timing instruments, chronometer trials were reduced to 30 days as to the earlier standard of 6 months trials when done by clock comparison.
I think it very exciting and interesting that even after studying for many years we still have the opportunity to find more and more about these instruments to satisfy our appetite of information about these remarkable timekeepers.