The Web lists of Medal Of Honor recipients do not show a person with the last name of Larson. It is entirely possible that GSA made an error and sent the wrong stone. It happens.
The four VFW members are waiting to do what military protocol refers to as a “three volley salute” which uses rifles. A “gun salute” uses artillery pieces.
Because the VFW members number four, a three volley salute would be 12 shots. They would need four more members for 21. Typically the rifle squad would be an odd number (3-7).