Interesting question, but one of which I`m not sure there is a single answer.
The Diehl Induction Lamp is an extroadinarily scarce lamp, only two are known to still exist in the world as far as I know. One was at Mount Vernon, the other in the hands of a certian mr Westlick, last I heard.
But I`m willing to bet that in this big old world of ours, there are some types of bulb of which there are no others in existance. Either they were created as unique one-off prototypes, or they are the only survivor from a small obscure company who never made that many in the first place.
As far as their value goes, well famous examples like the earliest Edison, Maxim or Swan lamps- and of course that Diehl lamp- would fetch the most. An oddball lamp from some company no-one`s ever heard of, or a plain looking prototype that was hand made at the labs of Osram, GE or Swansea University (I know James has made one-off bulbs before now!) may not fetch that much. It`s a bit of a pity, but I reckon that`s the way it would go.
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