Jonathan Veley wrote a great aricle on "Dots and Spots" (it's now in the 2'nd or 3'rd volume of the Leadhead pencil blog book) with a second article on the subject here.
According to the article it seems that both Mentmore and Sheaffer got their inspiration to add the white dot from Dunhill, roughly at the same time.
I believe Sheaffer did not sue Mentmore because the Spot brand was small enough and far enough from the Sheaffer markets as not to arouse their attention.
Here is a picture of the other Spot I have in it's original box:
Got the GF pencil too:
Here's an ad of the Mentmore Spot
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