Leaving aside the fact the heap ( A.T.P ) combined every design flaw imaginable (possibly due to having a surplus left over from the 748) some comedian at Woodford / Chadderton thought it a " good idea " to use the same flap drive gearbox on the A.T.P....the oil replenishment plug was.......5/16 BSW.
They tended to become "a little smooth round the edges ".