There is a line of rocks running westward from Rock Island to Locke some of which are apparent in attached photo taken from the south west. This somewhat agrees with the mystery photo .
The Gilford tax card has a photo of the current two story structure:
http://gilford.univers-clt.com/view_...&series_card=1
The listed construction date is 1915.
About 10 years the Gilford Island Assoc picnic was on both Locke and Rock. We met the owners of Rock Is who said that the land had been slowly built out over many decades and the retaining wall installed. Every visitor to the island had to bring a bucket of soil or gravel.