I purchased two Pelikan 120 pens, one from the 1955-65 era and the other made by Mertz & Krell in the succeeding years, both set up as desk pens. Whether these were factory models or a very well-done field modification is not clear, but the Pelikan factory has definitely produced similar items, as this picture shows:
In both cases, the original clip and clip retainer were removed and a swivel mechanism attached to the cap. The pens themselves are normal pocket pens, so with a separate cap, one could treat these as convertible pens for desk or carrying. Neither pen had the actual stone base, but fortunately their swivels matched the hole diameter in both Sheaffer and Wahl-Eversharp bases, so I’ve mounted them on bases of my choosing.
First, here’s the earlier 1955-65 pen on my chosen base:
And here’s the Mertz & Krell version on a small base: