After two days of soaking and rinsing the base of the arm assembly in solvent, I think I finally have the ball spacer free.
I cannot see it, because there seems to be something keeping it down in the bottom of the ball race area, but I can hear it moving when I turn the arm assembly over and back.
I did not find anymore needles. Three was enough, I guess. But with the colour change in the solvent and with the amount of debris washed out, I would say there had been a lot of hardened grease containing dust and pet hair left after my initial cleaning.
I am still trying to understand the structure of the arm assembly. I suspect that these are the component parts--I offer an improved, I hope, drawing from that which I deleted from the post above.
A: Pot metal arm with steel base
B: 2 rivets for attaching auto-brake trigger to arm
C: Auto-brake trigger
D: Pivot pin set screw and nut
E: Pot metal support bracket
F: Steel bottom plate
G: 2 rivets for attaching steel bottom plate to pot metal support bracket
H: 3 mounting screws for attaching assembled arm to motor board
I: Ball bearing spacer
J: Flanged throat, press fit into bottom plate
K: 5 ball bearings
Items G, the two rivets, for attaching the steel bottom plate to the pot metal support bracket seem very flimsy, perhaps there only to hold things together to speed assembly of the whole machine. They add no real strength to the assembled machine; the three mounting screws hold everything together against the motor board and the cork gasket.
There seems to be no reason that two small machine screws could not have been used here in place of the rivets, except that doing so would have added to the labour cost of building the machine. The holes would need to be threaded, and the screws would need to be screwed in—much faster and cheaper just to pop two flimsy rivets in place.