Have a later version Metrogon.

Note the picture showing the rear internal and external surfaces that
face the film. The aperture stop and the internal surrounding rear parts
are WHITE.

I have never seen this before. What could the rationale be? Has the
manufacturer (Fuji in this case, I think) have discovered something
counterintuitive but useful?

See pictures he
http://www.digoliardi.net/metro/met1.jpg
http://www.digoliardi.net/metro/met2.jpg
http://www.digoliardi.net/metro/met3.jpg
http://www.digoliardi.net/metro/met4.jpg