I don't know what a banana slit is, but i need/want one! I think a Boston Shake is a milkshake with a sundae in it...
Update!! found this tidbit via this page:
On the West Coast at the Foster Freeze and DQ stands back in the 60's we would order a BOSTON SHAKE. That was a thick milk shake with a DQ soft-serve sundae built into it...looking somewhat like a Float but with the topping visible (hot fudge?) Usually the sundae flavor matched the shake flavor...Chocolate/chocolate or Butterscotch/butterscotch, but having a hot fudge sundae float in your butterscotch shake was a real treat too.
