We have two of these cards, and as usual with older postcards we need to explain the differences carefully.  One (in the scan) has all four corners intact.  It was definitely mailed, and has two different same-day 1906 postmarks from different locations, but no stamp–and no sign there ever was one.  There is some kind of pink staining on the reverse, less so on the front (Grade: 4, $6).  The other (not pictured) has bits of two corners missing, and was mailed in 1908 with stamp and full Boston postmark.  The writer compares this to Bug Light in Boston harbor (Grade: 4, $4).