The most used hidden printer in the world graded your tests, endorsed your checks, and ran the calculator on your desk. You never saw it, because it wasn’t sold as a printer — it was sold as an engine, built into somebody else’s machine.
One Cartridge, One Engine
This is the HP 51604A thermal inkjet pen — TIJ 1.0, launched alongside the HP ThinkJet on February 1, 1984. HP put the nozzles, the heater, and the ink into a single throwaway cartridge, then sold that engine to any manufacturer who wanted to build a machine around it instead of designing their own print head.
Where It Turned Up
The list of machines that ran on this exact cartridge: Scantron scoring machines, TeleCheck and First Data check terminals, bank proof and endorsing printers, Canon printing calculators, fax machines, Ultratec Superprint TTYs for the deaf and hard of hearing, TEMPEST secure printers, and Miltope rugged military printers. Different brand names, different plastic, same 51604A inside.
65 Million, By HP’s Own Count
HP’s own data sheet (4AA4-2721ENW, 2018) puts the delivered total of this cartridge and its direct successor at 65 million units worldwide. That number is the whole reason to ask whether this is the most used printer in the world.
The Verdict
It isn’t — and I show exactly where it loses. Part 1 builds the case device by device: the eBay box that said “Scantron ink,” the test day machine that graded you, and the paper trail from 1979 through HP’s first ThinkJet. Part 2 puts one 51604A pen through three of these machines, built 42 years apart, on camera.
Rob streams live repairs and builds every Tuesday and Saturday at 7pm Eastern on Twitch. His current series is Shelf Life — pulling broken hardware off a ten-year shelf and not stopping until he knows why it died.