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 sha