I haven't ever tried to put a 56LA on a model 71. If it will fit, then it will probably work. I thought the Marlin hole spacing was different but the difference is probably only the width of the arm that goes over the top of the receiver. It is probably a little shorter than the 56A.
For the "purist", the 56A is actually not the period correct sight for a 1956/57 gun. I believe you want the Lyman 66MA which is actually harder for ME to find than a 56A.
Last edited by Levers; 04/05/12.