Hi,I can tell you what the 902 letters are.The AAA stands for Antiaircraft Artillery and the AW stands for Automatic Weapons and BN stands for BattalionCAC stands for Coast or Coastal Artillery Corps.
Here is a little info on the 902nd:"Constituted in the Army of the United States as the 902nd Antiaircraft Artillery Automatic Weapons Battalion (Semi-Mobile) and activated 15 September 1943 at Fort Gulick, Canal Zone.Disbanded 15 April 1946 at Fort Gulick".
Campaign Streamers World War II American Theater without inscription
Decorations - None
Coat of Arms - None
Distinctive Insignia - None
I would think the SI is actually an S1 which logically seems to mean Seaman 1st Class.
I believe that in order to have military information notated on a headstone, it had to be proven somehow that they were in the military.So some family or the funeral home or the Sexton of the cemetery may have those records.
To try to find records, if you know what state and county they were living in at the time the war broke out, you could try contacting the County Clerk and the State Archives to see if they have any records.I believe that where they enlisted, they also had to go to log their discharge.
If you think they were receiving VA benefits, you may be able to find out some information from them but you will need their miltary numbers.
Another option is the VFW.You might try them in the city where they passed.
Hope this gets you started along with Mr. Fronzek's information.