I checked the S-T parts catalogs for the Tr6 (with A type OD), Tr250 and the Tr4A.
It looks like the OD filter attachment bolt/spacer/washer was deleted for those models.
The S-T parts catalog for the Tr4 does show those parts, but as it happens my '63Tr4 does not have the bolt, etc. But its OD does have the large drain plug with the spanner wrench slots.
It looks like the requirement for the filter attachment hardware went away with the change in the OD to the large drain plug with the spanner wrench slots. These ODs do use a pump body plug (p/n 513902) that seals the hole used by the older versions of the OD units to attach the OD filter element.
This type of large drain plug has a recess to accommodate three large rubberized magnetic washers and a center protruding boss into which the center hole in the OD filter element fits, this boss centers and locates the filter element for installation in the OD.
Looks like OD units with this type of large drain plug depend on the presence of the magnetic washer stack to ensure that the filter element seals at the OD end.
I don’t have an OD drain plug like CJD has illustrated in his post for comparison to the latter version of the OD drain plug.
CJD, does your OD drain plug have an internal protruding boss?
M.