My First Vacheron

I was set up at a local antique show and managed to snag this off another antique dealer. The dial does not suffer from chips or cracks which is great considering it looks like it saw lots of use.

Serial dates it between 1915 and 1920. The watch is a contract watch for the US Army Corps of Engineers. The contract ran from 1918 to 1920 for 5000 watches. From what I have found on line they delivered under 4000 of them and the large majority was for the single button chronograph.

As far as I can find the watch is 100% original and unmodified. I am interested in finding out more about the number on the back. I feel that there is some discrepancy in how the watches are numbered. While I understand there was a 5000 watch contract with V&C for Corps of Engineer watches there appears to be a letter requesting "half chronometer" watches from V&C in the total of 2000 as well. The letter states that they are to be numbered 10,351 up to 12,350. If the numbers on the cases on the image below are scrutinized there are cases marked with the same "property of Corps of Engineers USA" between 10,986 to 12,51X that are not V&C time only watches.

Would the US Army request multiple companies to use the same numbers on multiple watches of multiple makes and functions? If these numbers were used to record the issue of a watch then you could have 5+ people with the same watch number. This seems very poorly recorded and tracked on the part of the US Army. 

The watch needed some external and internal gentle cleaning. Ticking away happily on my desk.