Answer: sliderules

AnswerCrossword Clue
SLIDERULES

Obsolete calculators

SLIDERULES

Playground protocol?

SLIDERULES

Engineers' gear, once

SLIDERULES
Calculator predecessors
SLIDERULES
Early calculators
SLIDERULESInstruments used by mathematicians.
SLIDERULES"No pushing" and "Go down feet first"?
SLIDERULESForerunners of pocket calculators
SLIDERULES"Feet first" and "No pushing"?