It looks like the 671 had loose bearing between thrust washers. The 681 had the thrust bearing with the balls caged.
I would try to find all the bearings you can(large magnet) then divide them by half.
The 681 shows 5 caged bearings on each size so you should at least find 10 or more of them for your 671.
Get some white paper towels for your work area and study the diagram for placemet of the washers. Using the old one will work no need to replace.
Let us know what happens.
franktrain