Bob's generally correct, but the meaning if USWV is more specific. USWV stands for "United Spanish War Veterans." This was a private veterans organization, similar to the American Legion or VFW. It accepted men who served not only in the Spanish American War, but also the Philippine American War. It is not a governmental designation.
The local chapters had both a camp name, and a number (i.e. "Joseph Wheeler Camp 4")
The 28 may refer to a Massachusetts Camp number, but I cannot say for sure. There was no camp named "28th Massachusetts" and there was no 28th Massachusetts regiment in the Spanish American War.
There were some men who were young when they served in the Civil War, and old when they served in Spanish American War...maybe he did both.