It seems to me that the pattern of holes showed up on analog boards sometime in late 1991 or early 1992 around the time the Classic II came out.
Apple may have been using up older stock of case backings, sort of like how they made SEs in 1991 to presumably use up parts. The sound vents are found on mid-1992 and later Classics. My guess is that they started putting these in because of complaints that Classics aren't loud enough--put both a Classic and an SE on speaker volume "7" and you'll see what I mean.
The new question--was the concept of speaker holes thought of long before it was implemented? Could it be Apple got complaints after the first round of Classic IIs that the stereo sound should sound better from the internal speaker?
Apple may have been using up older stock of case backings, sort of like how they made SEs in 1991 to presumably use up parts. The sound vents are found on mid-1992 and later Classics. My guess is that they started putting these in because of complaints that Classics aren't loud enough--put both a Classic and an SE on speaker volume "7" and you'll see what I mean.
The new question--was the concept of speaker holes thought of long before it was implemented? Could it be Apple got complaints after the first round of Classic IIs that the stereo sound should sound better from the internal speaker?