In reply to Double-struck is the blackboard’s bold:
In a world of mechanical typesetting, a certain amount of improvisation was necessary to overcome the limitations of the physical media. I guess that Typ-It insertible type bars were a little before my time: I began operating dedicated strike-on machines in 1966, when the Varityper 1010F had been well-established for years, and the IBM Selectric Composer was just emerging. Fake bolding wasn’t necessary on either of these machines if you purchased a boldface type element.