The decals are the same as my 115. IIRC the 15 is an oscillating hook where the 115 is a rotary hook. Look at the hook (the part where the bobbin case inserts) while you slowly turn the hand wheel. That hook will continue to rotate in the same direction over and over again when it is a model 115.
My 115 treadle is my second favorite treadle machine and I don't think anyone could pry it from my hands. I recommend you keep and use it!