Yan Wo, you might as well ask "why is the gear change quadrant called a 'banjo'?" [useful dictionary definitions deleted] It's the swinging frame aspect that gives rise to the "tumbler" nomenclature. The handle of the feed select lever, of course, being the end of the swinging frame...
Thanks, sfriedberg. If I understand your first point, it's that the term might have just been pulled out of the air. However, using the term "banjo" to succinctly identify what might otherwise have to be described as the swinging end gear idler support arm, makes sense to me because the part does have a superficial resemblance to the musical instrument. Just as the "lighthouse" tool post superficially resembles its namesake. I could also have asked, "Why is a way called a way," or "Why is a spindle called a spindle."
I didn't search for "tumbler gear," so thanks for that insight. The dictionary definitions certainly support your second point, and it makes sense to me that South Bend uses "tumbler" to refer to the entire assembly: swinging frame, attached gears, and locating mechanism. Thus, the tumblers on the QCGB, the tumbler on the apron, and the reversing tumbler too.
So it is likely that tumbler gears were used in machines 100 or more years before South Bend made their first lathe.
But that begs the question, "Why did the very first person to use a mechanism with 'one or more idle wheels journaled in a swinging frame moved and clamped in position by the operator' call it a tumbler?"
Following your lead, I found many definitions of "tumbler gear," a few of which repeated the ambiguous phrase, "a train of gears in which the gear-selection mechanism is operated by tumblers." I thought I learned in grade school that you can't use a word to describe itself. The words, "operated by tumblers" in this very poor definition seem to contradict the good definitions you provided.
Thanks again for your response.
Jon