Yes they were specifically set up for the B-21. It would be dificult to install the subframe on any other tractor.The subframe runs from the back up to the clutch housing/engine where it becomes part of the loader subframe. The B-21 also has an additional hyd. tank mounted under the tractor to help with cooling. You may be able to install the BT-751 on the B-20 which is the older B-21 model but I have never tried it.
J.D.
