Front Cover of the Operating Instructions Friden SBT Electromechanical Rotary Calculator

The Friden SBT-10 was manufactured from 1959 through 1965. The SBT was a rather serious modification to the earlier Friden ACG calculator requiring the addition of a substantial amount of mechanics to add the ability for the number in the upper dials to be automatically transferred into the keyboard (called "Back Transfer") for re-use in calculations.