What is the part number for the Power Take Off cover on a 1956 F40.
The original part number for the PTO cap is 180313M1.
The F-40 PTO cap is the same as the one used on the TO-35, MF-35, MH-50, MF-50, MF-65, MF-135, MF-150, MF-165, MF-175, and many later MF tractors. The shape of the cap has changed slightly over the years with the first version having a more rounded end. Later versions have a flatter, less rounded end. Still later versions have a stepped down body design. All of these caps have the same thread, and should fit your F-40 though the first version with the more rounded end is the version that came on the tractor.
The internal threads on the PTO support housing are sometimes damaged, but can usually be repaired by using a thread restoration file on them or by using valve lapping compound on the threads of the cap and support to restore the threads. Be sure to remove all traces of the compound when you’re finished lapping the threads if you use this method. The abrasive will tear up the PTO oil seal if it migrates down the shaft. If the threads cannot be restored you will have to replace the PTO bearing support.