The cap is not Sterling silver, obviously, but could it be worn Vermeil? I do not know which markings Vermeil caps had.
But you do not expect a silver clip and silver lever on a Vermeil cap. I have never seen a Skyline with a silver lever anyway.
The cap does indeed bear the metal content imprint consistent with Vermeil.The seller notes that the "lines" on the cap are not "embossed." And there is a spot near the lever at the top of the cap where traces of gold apparently remain (the photos could be deceptive here). Taken together, perhaps this does indicate a worn/modified Vermeil cap. According to David Nishimura in a thread from years ago:
during the war quite a few Skylines were made with gold filled over silver caps, but they were invariably marked with the gold content. Polishing the gold surface down to the silver is possible, however the metal content stamps would remain.
If the lever, clip, and clip hardware were also Vermeil on the wartime pens, perhaps these too have been polished down to the underlying sterling(?) The other possibility is that this was a factory job, resembling a Parker 51 wedding band cap. But like I said, I've personally never seen any other examples of such a beast.