The part number for the SL-1301, 1401 and presumably the 1501 (though I couldn't find that one) tonearm rest is SFPRT17001K. That's not the part number for the SL-1400 arm rest So my earlier post was incorrect. The basic casting for the plinth on the "MK1" versions of the SL-1300, 1400 and 1500 is the same basic casting used for the SL-1301, 1401 and 1501 but the arm and arm base for the two series are entirely different and that's where the arm rest is attached.
So, the only "drop-in" replacement will be the arm rest for the SL-1401 and its siblings. But as bigx5murph observed, other arm rests can be made to work, either by modifying the arm rest or the turntable.
John