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Here's photo off a complete set for sale advertised as '68.
The one that you have pictured is different than the one that Brett posted. There appears to be a "dimple" sticking inward in the middle of the wide side of the opening where the mast fits in and the opposite side looks to have a slight curve to it, as opposed to being straight. Also, the base outer perimeter on the one you show is curved on all four sides, whereas his image shows them as being straight. Here's some of the ones that I found in a box.
You might post a good pic of the other end of the jack shaft since the other end is what is supposed to slip into the trapezoidal base hole.
I also noticed in Bob's pic (showing a bunch of bases) that the LENGTH of the trapezoid hole seems to be different on some....some openings being longer than the others. I'm not talking about the solid "T" profile shafts/bases since those are obvious.
Perhaps a couple of good measurements.....front to back "length" on the shaft end and the two "widths" as well. Might help determine which base style the shaft will fit into (longer trapezoid or shorter trapezoid openings).
I dug out these for better pics & comparison
note the depth of legs to the actual mast socket. And the height of the legs compared to the 72, pierced hole in 68-9 base and the snipped corner
I hope these help
Brett