I just bought one at a garage sale. Oddly, it is not mentioned in the
instruction book that came with it.

All I can figure is that it goes on in place of the blade; the arbor nut
goes into the head and then the head is screwed on with the nut in place. I
haven't tried it yet, but it looks like the arbor is too long; does it
require a spacer?
Thanks.

Don't know exactly why I want it, but for $2...