There are at least 2 Service Bulletins addressing the engine to pump coupler on D series tractor.   #217 from 1978 specified a kit (obviously NLA)It included a new mounting bracket but also a splined washer (#106641 NLA).  Also 4 washers #920008.  Research reveals the 4 plain washers are nothing but SAE5/16".  However I can't find any source (McMaster/Fastnal) that has splined washers.   The second SB from 1981still uses the new washers and splined washer,but instead of greasing the pump shaft and coupler (earlier SB procedure) you seal them with Loctite 680. Which I have coming.   Question?  Does any one have the splined washer or at least know what it looks like?  If I don't use one is there a risk???