Just to close the loop on this, I'll report that my mast, the original Lefille, has an access plate only on one side. Removing this plate reveals a round, hollow tube (about an inch diameter) upon which the three forward sheaves are set. The back three sheaves have the same arrangement.
We drilled a hole in the opposite side and managed to tap out the tube, then take out the sheaves. Tomorrow we put it back together, along with a new access plate for the new hole. The back sheaves worked fine, so they remain in their original situation.
Andante PSC 34 #276