Visit or contact the National Archives in St. Louis and you should be able to get a copy of Garza's personnel file. It's normally $70 for them to photocopy the file and mail it to you. If you or a friend/relative can visit in person (appointment required) then you pay exact photocopy costs only (25 cents/page). Some of the personnel files are pretty extensive, some are pretty slim.

Alternatively, in the Washington DC National Archives they have Retirement Record Cards on file - Record Group 26 Entry 7(A-1) - for keepers who served during that time period. It's a single card with brief info, but will list dates he was assigned, transferred, and/or promoted. I don't know if they are able to find it, photocopy it, and mail it to you, but you or a friend can visit in person if you go to DC.