Dating the start of the 120 Wurlitzer:
I just worked on a 120 with serial number 10034. Grey sandstone/flecktone style. The harp in it was early. It had two bricks at the top, and the masking tape on the back of the harp said "1/16/57." That's the earliest date I've seen for a 120, so that is interesting in itself! My timeline has 112A's, the prior model, seemingly still the current/most recently released model in October 19, 1956 (based on note #7). We knew already that there were 120s by May 21, 1957, because of Wurlitzer's note # 9, which mentions them.
I am a little suspicious, though, of that serial number. The earliest 120 serial number I know of is 8999, which seems to be a cream-colored wood-textured look, with a picket fence style music stand. (there could be earlier ones.) Might they have made 1035 of them in just a couple of months? I think it is possible that this is an early harp Frankensteined into a body from the following year....or, that someone wrote "57" when they meant "58"...a common January error.
#16217 is from November 28, 1958 (similar dating, using tape on the harp). Assuming there aren't gaps in the numbering: Between November, 1956 and November, 1958, #16217 - #8999 would be 7218 120's produced, or 300 produced a month. In fact, if they started making these in October, 1956, that would be right on for the dating of the January 1957 Wurly, and its serial number.
Of course, there could have been many more before #8999. The last 112A I've seen online is #7200. 16217-7200 = 9017, or 375 produced per month in 2 years.
It is also possible that different body styles of 120's were being produced at the same time, with ranges of numbers allotted to them. So, the cream-woodish ones could have started at, say, 8500, the sandstone/flecktone ones could have started at 9500, and the brown mahogany/cherry ones (which, Steveo?) could have started at 10500....which, given some evidence I've seen, may be extremely accurate. If so....the numbering might not be strictly chronological.