I've heard on the grapevine that IBM 3274 uses an IBM proprietary microcontroller architecture known as "universal controller" (UC) – https://bitsavers.org/pdf/ibm/microcontroller/universal_cont... – but not sure how true that actually is. In the absence of written documentation–which probably exists somewhere... but nobody has scanned it and posted it online–this stuff is often just people relying on their decades old memories, and sometimes things get garbled with time. Or some people may even have been confused to begin with.
