Does anyone know whether 5- lens 240 tail lamps are interchangeable with the 6-lens type?

B.C. is correct. If you've got 5-panel lights, you can open up the hole to fit 6-panels, but unless you weld in some metal, you can't go from 6 to 5.
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)

I did see a 1983 or 1984 sedan at a yard once that did have five-panel taillights. The lenses were actually in great condition. Upon inspection, it seemed that the excess space that the 6-panel lenses would have taken up was filled by a carbon/smoke-colored piece of hard plastic/plexiglass(?). This plastic was visible from the back. I didn't open up the trunk to look at it, but it did look a little different. The car didn't have anything else out of the ordinary, so perhaps it was rigged by a mechanic who didn't know the difference between the 5-panel and 6-panel lights.
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Regards,
Eric Staufer,
'89 244DL 118k
My 240 Page