Pelikan 400 from 50s. tortoise grey ???

[MarcinEck]

I recently bought a Pelikan 400 tortoiseshell pen from the 1950s. Its cap, captop and piston knob are in an unusual grey colour ....

On the cap ring there are no inscriptions. Above the piston knob engraving GUNTHER WAGNER PELIKAN. Gold nib Au 14 ct, Pelikan in filled italics, 585 14 KARAT, OF.

I attach photographs illustrating this pen accompanied by 2 brown 400s from the 50s and 80s and 1 black-green Pelikan from the 50s.

What is your opinion on this?
I asked a question about this pen to many outstanding specialists from Poland, the USA, Germany and France:
The answers are ambiguous and often contradictory:
1. „… My first thought was, that strong sunlight has bleached the resin over the years. But in this case, you normally see swirls from the moulding process.
Your grey looks homogenous. …“

I'm relatively sure, that there was no grey/tortoise model. Maybe a factory employee makes some proofs. Don't know. …”

2. „… There was no such official release that I’m aware of…… Some of the early tortoise 400s were made form a plastic that reacted with light to form a gray haze over the surface. Perhaps that is what we are seeing here. That’s the only explanation that I can fathom though I’ve never seen one of those in person to comment whether or not the appearance of yours is consistent with that. … „

3. „…simply brown plastic degradation/discolouration …“

4. « … What a great find! He looks great!
„… I've never seen one with the cap and piston knob in grey, it's really rare! And since this is a uniform colouring, it obviously can't be discolouration. … »