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This is a set of 8 Pennsylvania Railroad Position Light signal heads. These are near scale based on PRR drawings. The target diameter is 15.2mm , HO scale 4 ft. 4 inches.
They are intended for tiny LEDs so the light hole diameter is 1.6mm. The intention is that the back square part of the LED looks like the light enclosure on the back of the target.
Note: These are very thin and fragile. Handle with care.