Here on page 402 of the Montgomery Ward 1969 Christmas catalog is a special type bar print head for the company's 'Signature' line of electric typewriters that permits special characters for engineering, mathematics, international language, and medicine. If you look down the page here, you'll see a red key on the keyboard that activates the Dial-A-Type type bar. The 'type bar,' BTW, is the arm with the embossed character on the end that strikes the inked ribbon to print a character on the paper. Although I could not find any instructions online, it appears the two rows of characters are accessed with the normal and shifted key presses, and a twist of the Dial-a-Type head selects which pair of characters is used.
The $12.50 price for the Dial-A-Type in 1969 is the equivalent of $80.84 in 2015 according to the U.S. Bureau of Labor Statistics' inflation calculator. Therefore, if you needed to have all four Dial-A-Type heads on-hand, it would cost you $50 in 1969 dollars, or a whopping $323.36 in 2015. That really make you appreciate the advent of software-based typewriting!
Here is another website with an actual photo of a Brother Dial-A-Type key.
Posted December 28, 2015