[Capt. Tom]

These machine-counted ballots with the unchecked Senate boxes could be culled out after the voting ended and fraudulently filled-in beyond the view of poll watchers.

That method goes back a long way in our history.
In the late 1800s the paper ballot counters would have a piece of lead from a pencil under their thumbnail, and when they spotted an unvoted block they could fill it in for their candidate with an X using their thumb.
You could also void a ballot by voting for a second candidate, to take away votes from a candidate you opposed. -Tom

[Sacajaweau]

interesting