I still don't know what that "thingy" is, but that Shuttlecraft, you
FORGOT to post that, it's awesome!!!
Well, you asked an unanswerable question.
The ribbed section is the interconnecting dorsal of the Excelsior (between primary and secondary hulls). Note the "heat-sink" style of neck detail that ILM was in favor of suggesting, at the time (this styling will reflect in the Bird Of Prey warp radiators, the Grissom nacelles, the Merchantman pylon ends, and the 9 ft Excelsior model; all of the same movie).
In the "neck" were these projectors (See image 1 from the filming model). That's what you are looking at in this replica build. They were never explained on screen. Many have assumed, for good reason, that they were torpedo tubes, as this was consistent with the 1701/1701-A vessel designs. The photorps were located at the bottom of the Interconnecting dorsal of the Constitution II design. The problem is that photon torpedoes were seen fired from the sides of the secondary hull, several times, in ST-VI (See image 2). Not from this twin interconnection dorsal area. That alone wouldn't negate that these were intended for that purpose, as Sulu could have fired tubes one and two, and not fired what would have been from the four more in the interconnecting dorsal, if he wasn't motivated to do so, but adding to an argument that they are not photon torpedo tubes, is that the projectors don't actually look like torpedo tubes, which have a light strip-faring around them in both 1701 (see image 3), the Reliant (see image 4), and the NCC-2000 (see image 5) filming models [see the filming models detail on both]. These "thingys" are in the right place for a dual bank of four photorp tubes, but not sculpted similarly. Are they different looking torpedo tubes?
Some have suggested that they were megaphasers, and they do resemble the tip-ends of the Reliant model's phaser canons. The problem with this suggestion is thier placement at the top of the interconnecting hull; they would be better placed toward the bottom (as will be explained in a minute). Others suggest that they are a form of the sensor system; either targeting sensors or even the space energy sensors that are missing from NCC-2000 hull that are present on 1701 or 1864 filming models See image 6, at the sides of the hull, and at the bottom of the front of the secondary hull). The projectors in the Excelsior neck look differently than the space energy sensors of the 1701 design, and they would be much larger than those on the other ship designs, but that's another possibility.
What makes these even more mysterious is that the 'line of sight' positioning in the neck, so high in the dorsal, makes them a challenge as sensors or as weapons. The projectors are too high on the model's neck (unlike in the 1701 model), making a direct projection for forward action, lining up into the lower part of the primary hull bottom, right into the space of the main sensor. This would mean that photon torpedos could only fire downward, or strike the hull of its own ship, with the same effect for megaphasers, and as sensors, they would project into the main sensor fields of projection.
So what are they? Well..."thingys." They don't really fit anything quite well as anything suggested, and reflect an artist error in ILM when designing the model. Not the right shape for torpedo tubes, even when compared with the consistency of tubes (as with the filming evidence on the sister-ship models), not well positioned for phasers or torpedos or even probes, and not even positioned well for targeted sensor projectors (whether energy or weapon's targeting), running the sensor beams into the hull or field of main sensor projection, instead of beyond the ship. Complicating things, was a FX filming error in the film "Generations," where the same model, dressed for 1701-B, suggested a work bee pulling a cargo train out of the aperture around these projectors; clearly not possible given the detail of the interior of this model, but launched a number of un-informed website comments and speculation.
As of today...they remain projector "thingys"...and with Bad Robot's poor Hx. of continuity, or even comprehension of consistency value, they may remain so identified...