Thank You for the pictures.
The stock mounts and has the same shape of a monte carlow stock. It is an exstention of the stock and not a pistol grip or thumbhole through the stock body. Your hand gripping the U-15 stock would be in the same position as a standard straight back stock like a 1903-A3.
PLEASE WEAR EYE PROTECT & PROTECTIVE GEAR IN SHOP!!!!!! You can order another part from from manufacturer, but you can't order another finger or eye from your mother & father.
***This DOES NOT constitute GOOD or SANE legal - professional gunsmithing - psychiatric MD - tax - accounting -gardening advice. Please contactact qualified a professional in their repective specialties.*** AWHHH go ahead and mix match specialities that could be funny!!!!!