The 1961 and early 1962 manual trans bell housings were only made for the three speed trans. The back face and ears are too small to add 4 more holes to fit a 4 speed. The later 1962 and all 1963 bell housings were drilled and taped for a three speed AND/OR a four speed. They have 8 taped holes on the mounting face. If you have a three speed hooked up to a bell housing and were wondering if the bell housing would accommodate a 4 speed, you would be able to see the open 4 speed holes outside of the 3 speed trans.