And NASA said: Let us create a swing arm, more beautiful and more glorious than all the others, with pipes, wires and cables, that it may feed the second stage and be well pleased by it.
And the engineers went and created a swing arm that far surpassed all the others. It was so full of pipes and retrieval mechanisms that they were constantly expanding and improving it.
But since they had completed it, a scream and lamentation among the model builders began: How can we ever create a model of this truly magnificent swing arm when it is so incredibly complex?
But NASA said: Behold, there are ... uh ... plans, and hand sketches, and ... uh ... pictures ... and at all.
And the model builders went there and cried bitterly.
Service Arm 4 - S-II Intermediate
The genesis of the Service Arm 4 - S-II Intermediate - could have happened in this or a similar way. The reality, however, was far more difficult. The Saturn V was too heavy by several tons. As the development of the first and third stages was already so advanced that no more changes were possible, the thankless task of a sustainable reduction diet fell to the team for the second stage. The whole story probably happened like this:
Old Experienced Engineer: Oh no, not again ...
Young Snotty Brat: We build a single tank!
Old Experienced Engineer: Muahahahaaaaaaahahahahahaha!
Young Snotty Brat: Seriously. We save the few tons easily!
Old Experienced Engineer: *gasping* And the liquid oxygen and hydrogen we then pour together into the tank, right???
Young Snotty Brat: Well, you could put in a kind of false ceiling ...
Old Experienced Engineer: Deliver me from this moron!!! The oxygen would freeze at the common bulkhead. Do you hear????? FREEZE. STIFF. O-XY-GEN-ICE. And do you know why? Because the liquid hydrogen is 100° cooler. Do you understand that, brainy?
Young Snotty Brat: And if we simply insulate the false ceiling?
Old Experienced Engineer: Listen my boy. An insulation over a gradient of 100° per inch must first be invented, ok? BECAUSE IT F*CKING DOESN'T EXIST ON THIS BLOODY EARTH!
Young Snotty Brat: Well then we just invent it?
Old Experienced Engineer: ... the pills, quick ... he has gone mad ...
S-II inboard profile: Apollo Systems Description Vol. II 1964, p. 25-5, Courtesy of NASA
Because the original two tanks were pushed together into one, the second stage was not only lighter, but also more than two meters shorter. This in turn had an effect on the swing arm - the center and rear interfaces moved so close together that no two swing arms would fit between them. So the intermediate swing arm became the AFT swing arm as well, and an auxiliary swing arm was attached underneath, which only allowed access to the AFT area, but did not lead any lines to Saturn V itself.
Stage II connected to the LUT - view from below
So here's the new swing arm, and yes - the second stage is finished as well. The main work was the J-2 engine, where you can finally use your fine motor skills to the full. But I actually thought about the possibility of building it, and so I added an artificial level, which doesn't exist in reality:
Design of the J-2 engine model
In the Rhino 3D picture you can see it faded in. The error is about 0.2mm, so it's just tolerable.