In perusing The Old Outboard Book by Peter Hunn, I was able to find this motor in The Back Rack chapter. It is indeed a Neptune product "with a blue and yellow logo on a red fuel tank", per The Old Outboard Book. Interestingly, Mr. Hunn only references a motor called a "100" and opines that it looked like a 1.5 HP model 17A1 which would make it a 1947. While I can’t see your picture clearly, there appears to be a number on the back of the tank. Can you post a picture of that, or tell us what it is?
FYI, Gambles stores were better known for selling Scott-Atwater motors branded "Hiawatha" outboards in the 1950s.