score:3
This is an index card from the 'United States Veterans Administration Master Index'. The collection from 1917 - 1940 is available to search and view online at FamilySearch.
From the linked description for that collection, the following codes are listed:
An example of a completed card can be seen here:
Your surmise that Sn was the serial number is correct. This can be seen in the example of Clarence Thomae, from the example above, by examining extracts from his New York Abstract of World War I Military Service:
and his application for a Purple Heart:
where we see that his Army serial number was indeed 1,209,111.
This series of blog posts by the professional genealogist, Janice M. Sellers, includes the pro-forma postcard Form 7201. Based on that, I think that the first code, C (or XC in the example above), was the claim number assigned to that individual.
In the example of Clarence Harry Thomae above, the claim number would thus be XC 248 113.
Upvote:0
This document has the exact meaning of each of the code letters on the card https://www.archives.gov/files/calendar/genealogy-fair/2018/6-csar-handout3.pdf