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Here's a detail part for the De Agostini 1:2.3 (advertised incorrectly as 1:2) Artoo model kit. Restraining bolt version 3 – open front and a flat back.
Personally, as a supporter of droid liberty, I feel rather tainted making a replica of this product. But if you want to depict a cruelly enslaved Artoo unit, with a restraining bolt attached to its chest by vile baddies, then this is the part for you.
The real props.
The current theory in the builder community is that the actual restraining bolt prop was a repurposed old-style Arri camera lens, and that's what this replica is modelled after. It also includes elements of the restraining bolt attached to the ANH-era Artoo unit that's making the rounds with the Identities exhibition. I photographed this prop in London, and relied on that for details.
That particular Artoo and bolt appear to have been used only for pickup shots done in California (the shot of the droid merchant zapping the restraining bolt onto Artoo in a shower of sparks, for example) and thus may not be identical to the bolt used in the droid auction and garage scenes, which were filmed earlier in Tunisia and England.
In the film something resembling a carriage bolt and possibly a washer are installed at the end of the restraining bolt on the two-leg Artoo, and so you might want to add those. If you prefer, I sell a version of this bolt which includes those parts.
Now, there is an obvious continuity error in the film – the three-legged Artoo appears to have a restraining bolt with no visible carriage bolt, and so looks like this version of the product. But it only appears extremely briefly in the movie, and from a distance.
The prop on the Artoo in the Identities exhibition has what seems to be a circular piece of black construction paper or something similar stuck in the back of the lens opening. I've made a slight recessed area inside the restraining bolt housing if you want to put such a black paper circle in yourself.
This version of the bolt has a flat back, the way the original prop presumably had. I've added a three-sided crossbar for ease of attachment. There is also a 10mm hole in the crossbar, to accommodate a round rare earth magnet if you wish.
However, if you want one with a curved back that would conform neatly to the curvature of the barrel body of the droid, I have another version available. In fact, I have four versions of this prop: