This is an early design of the Monkeemobile by Dean Jeffries. It included a trailer that would open up and form a stage. The idea was that The Monkees would have their instruments in the trailer and could stop at any location to perform on a moment's notice. The trailer was never used as the idea was later scrapped altogether.
Here's a passage from Micky's 1993 autobiography, I'm a Believer, about the original setup of the Monkeemobile (thanks to Naked Persimmon!):
One day, just before we were to begin shooting the series, we were told that the ‘Monkeemobile’ was finished and was being delivered! I was thrilled. I’d often dropped by Dean Jeffries’ auto shop to watch the work in progress and had been waiting with bated banana breath for the debut of our super-duper custom Monkeemobile. The basic body had been a 1966 Pontiac GTO, but by the time the automotive artists had finished their transfigurations, there was little left that that anyone at GM would have recognized."
Cut To: Ext. Studio Lot—Day
The four MONKEES, the production staff, the crew, and the executives are all gathered in the parking lot of the studio for the big moment. Suddenly, someone shouts!
SOMEONE: Here it comes!
Sure enough, down the street we can see something big and RED rolling through the guard gate. As it gets closer we can hear the RUMBLING and rocking of the powerful engine; see the sleek, low lines of the customized body—see the sun REFLECTING off the bright chrome header pipes. An enormous high-performance blower sits on the hood and shrieks.
BLOWER: (shrieking) “Here I come, screaming down the street…”
DEAN JEFFRIES, the designer, is piloting the great beast and pulls it up to a stop in front of the waiting Monkees, who gather around like kids at Christmas. They’re ogling and drooling over the plush tan upholstery and the deep red pearlescent paint job when Mike notices something attached to the back of the car.
MIKE: What’s that?
Everyone looks over to where he is pointing. In all the excitement, no one has noticed the low, windowless, wooden TRAILER that is attached to the back of the Monkeemobile. The Monkees approach it curiously.
MICKY: Hey, what’s this?
PETER: It looks like a big doghouse.
The executives and production staff shift about nervously. It’s obvious that something’s up but no one is brave enough to come clean. An uneasy silence fills the air. Finally, a junior staffer is prodded out and steps forward; the sacrificial lamb thrown to the wolves.
STAFFER: It’s a surprise.
MIKE: (suspiciously) A surprise?
STAFFER: Yeah. It’s great. Look.
He leans forward, grabs the handle of a big LEVER that’s sticking out of the “doghouse,” and pulls it. Instantly, the sides collapse, swing out to form a ramp, and the roof folds back to form a sort of backdrop.
STAFFER: (enthusiastically) See? It’s a stage!
DAVY: (incredulously) A stage?
STAFFER: (nervously) Yeah…A stage. That when you arrive at, say, the grand opening of a supermarket, you can…
That’s as far as he gets. As if on cue, the Monkees exchange glances, turn on their heels, and march off. They manage to get about ten feet away before they burst out into hysterical laughter.
We must have laughed for two days. Needless to say, we never saw the Monkeedoggiehouse again. I’ve often wondered whatever happened to it.
- Micky Dolenz, 1993
From I'm a Believer: My Life of Monkees, Music, and Madness