Simon,
The Cibie units have a small square in the center of the orange lens that's different from the rest of the lens. The Siema lens is completely uniform. The lens in the photo above is a Seima and I believe they are what DeTomaso used OEM. I bought a set of these lights from DeTomaso years ago and they were made by Seima. The GT5's and GT5-S's I've seen also had Seima lamps. Unless you were looking for it, most people wouldn't notice the difference. The two are completely interchangeable which is good because the Seima's are extremely hard to find.
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