Elevatorman, are you gone now? Did I miss you by only a few minutes?
I searched for your serial number 29754 on the IPSND.net and the closest I found was 29755 for Gottlieb's 1960 Kewpie Doll which came out a few months after Dancing Doll:
This number 29755 is from the old Pinball Pasture website. I have their data to corroborate this. So that is the nearest number *above* yours. The nearest number *below* yours is F29620, also Kewpie Doll:
So, I'd say you have a Kewpie Doll lower cabinet.
Here is a picture of the Kewpie Doll cabinet side, a woodrail:
Interestingly, the 1960 game Flipper, where production switched from wood rail to metal rail, was the next game to come out after Kewpie Doll.
Gee, in support of the former owner's story, if this was Gottlieb's experimentation for a new metal side rail, it fits that they'd do it on a game closely preceding Flipper. But, why wouldn't they just do it to an empty cabinet? Or, if they wanted to see how it would look in general, why not just use the fully equipped Kewpie Doll production game that would already be inside that 29754 cabinet? Why insert a Dancing Doll playfield (and backbox innards) into a Kewpie Doll cabinet? Three production games came out in the few months between Dancing Dolls and Kewpie Doll: Captain Kidd, Spot-A-Card, and Melody Lane. Why Dancing Dolls?
Well, maybe the Gottlieb executive who took this home to his kids preferred the game Dancing Dolls.
These are speculations. I'd like to hear the details from the former owner, but maybe that opportunity has passed?
Your metal side rails are wider than what Gottlieb used, once they went to metal side rails in production, shown here:
The Gottlieb production metal rails curve around the flipper button. Your rails do not curve but have a straight edge at this end, and they each are nailed down at this end with three nails that, to my eye, do not have symmetry when you compare one side to the other. Is this operator's installation imperfection, or factory experimentation?
Your metal rail on the left side, where it ends under the backbox, looks like a squared cut, while the right rail ending under the backbox looks like its cut an an angle. Am I seeing this correctly?
Can you post a picture of the full cabinet front? Also, do the inside walls of your backbox have any paper tags affixed to them, and what do these tags say, and how are they affixed?