Is this an old original rifle or a fairly new one?
My little old book called "AMERICAN GUN MAKERS" by L.D. Satterlee and Major Arcadi Gluckman shows several Robbins and a company called ROBBINS & LAWRENCE.
The individuals shown are:
Robbins, Tooga, Pa. Flintlock maker
Robbins, W. E. Manesburg, Pa. Maker of percussion rifles.
Robbins W. G. Unidentified. Maker of percussion rifles.
Sorry there isn't more information in my limited supply of gunmakers.