I have seen this building referred to as the Grand Army of the Republic Memorial Hall and then later as Veterans Memorial Hall. From what I can gather, it was in active use as a veterans’ hall from at least the 1880s through the 1920s. The building still stands on Main Street.
Note the Sandy Bay Cafe to the right of the hall.
The card is postmarked 1911. No publisher is identified on the reverse side.
Vintage Rockport 
If would be fun to go back in time to see these places. Rockport is so bland now.
I was in that building a few years ago. I happened to attend the Christian Science Church. The inside looked to have a round ceiling like a army barracks. I wonder how much change was made from when the Gar left to the church moving in.
I think the building to the right became Cooney’s Market, and to the left, Tuck’s Pharmacy.