Where is that center inner ring from? Was it kit-bashed? Or machine-tooled?
I believe it was kit-bashed. I believe this for a couple of reasons:
1. Because whenever ILM could find the shape and size they were looking for in a model kit, it saved them time and money on machine-tooling.
2. Because on the actual model (unlike the DaveG perfectly-fitting 3D prints shown above), the center ring is slightly "off" of being perfectly flush with the inside of the Sealab Crane Part #59 double-sided/mirrored nurnie, as seen in the detail picture below of the Alan Ladd rear vectral
3. Because I believe I have found the part.
4. And because this part is perfectly "mismatched" to the Sealab nurnie in the same way the original was.
Here is the part on top of DaveG's 3D file printed equivalent:
Here is DaveG's 3D-file printed ring piece on top of the kit bashed part
Here are the two parts side by side.
Here is the part fitting "perfectly" inside DaveG's 3D vertical rear vectral fin, overhead view.
Here is the part fitting "perfectly" inside DaveG's 3D vertical rear vectral fin, side view.
Here are the measurements of the part, roughly 38mm inner diameter and 40mm outer diameter, but not "exactly" these numbers:
Inner diameter 38.62 (but don't take this as gospel, as I'm measuring a flexible part and each time I do it I get a different number, so even this is a "pretty close" guesstimation)
Here is the outside diameter:
Outer diameter is 40.54 (but again, don't take this number as gospel). The key is that it is pretty much 38mm on the inside and 40mm on the outside, with roughly 1/4 a milimeter (0.25mm) of wiggle room (i.e., the limitations of either my tool or my measuring abilities)
Here it is fitting inside (but just barely) into DaveG's 3D-printed horizontal rear vectral fin:
But here is where the part does not fit "perfectly" into DaveG's 3D-printed part, as the vertical ventral fin is slightly too "tight" to accept the kit bashed part, despite fitting perfectly into his other 3D-printed part: the DaveG part is a few mm shy of the full width of two Sealab Part #59's abutted to each other, which can be accommodated for inside the rear ventral ring (modified L'eggs Pantyhose container) by simply cutting off those same number of mms on the outer legs, as shown by the Gundam marketed pieces on the lower assembly in the photo below
So here it is sitting at the midpoint inside another version of the horizontal rear vectral fin, with plenty of wiggle room (but perhaps too much by 1-2mm)
So now here it is on the center part of a newly created horizontal rear vectral fin (in layout form, not yet glued together), which I believe is closer to the real original nurnie:
So where does this mystery greeblie come from?
So glad you asked...
The Entex 1/16 the Hon. C.S. Rolls 1908 Rolls Royce Silver Ghost "balloon car" released in 1975.
Sprue G, Part #1 (10 of them per model, or 5 Y-Wings worth) top side:
Sprue G, bottom side:
Close-up shot, top-side:
Top side is "pretty sharp" and "machined" looking on its edges, while the bottom side of the nurnie is softer, more rounded. So there are some discrepancies, as in many photos it looks like it's sharp and machine-edged on both sides.
But it doesn't look so sharp-edged on this archival Y-Wing, however, does it.
That's actually a trick of the light. In the Chronicles book, the lower left quadrant of this circle piece makes it clear that this is a sharp front and back edge, with a longer horizontal surface than front/rear surface area, which is precisely what the kit part has as its characteristics.
I take this to be the authoritative confirmed donor greeblie on Red Jammer, Gold Leader, Gold 2, and Gold 5. I am less certain on Gold 3, but this is only because I can't seem to find any conclusive pictures of this area on that particular model.
You can also get the kit re-issued later by Bandai in the 1980's, and much more common on Ebay.