I've assumed that the two M Shell rifles included in Shumway's Longrifle Articles, Vol 1 (one of which is pictured again in his Pennsylvania Longrifles of Note) were made by Martin Jr. However, we all know the risk of such assumptions...
In researching the Parthemore genealogy (Martin Jr.'s wife, Catherine Parthemore), there is mention that Martin Sr. was a gunsmith, in addition to Martin Jr. The Military Milestones of Old Paxton Township, 1715-1946 includes both Martin Sr and Martin Jr as "rifle makers".
However, as far as identifying rifles by both Martin Sr and Jr.....boy, that would be great!
Bill