Despite a painful familiarity with filling out Scantron forms, I’ve never seen the machine they were fed into. This video shows how they work, how the answers were programmed, and dives into its internals.
If you just want to see the guts of it, skip to about 14:30
Probably a pretty common scenario, I’d guess. Apparently Scantron had an interesting business model where they basically rented the machines to the schools for free as long as they bought certain amounts of the official Scantron forms.