A discontinued pen from Platinum’s ReCelluloid range of, well, celluloid pens.
From online searches, this pen was seems to date from the mid-1980s to mid-1990s. Lambrou’s “Fountain Pens of the World” lists similar pens (going by nib design) from around 1992.
The celluloid used resembles the striated celluloids used for the Parker Vacumatic:
It even has semi-transparent strips alternating with opaque, slightly pearlescent ones:
Platinum makes the ReCelluloid pens by rolling thin celluloid sheets, rather than by turning celluloid rods on lathes. The former saves money and manpower, but the pen ends up with a distinct line where the edges of the sheet meet.
This pen has a different feed (left) from current production pens (right). Could it be ebonite?
Its music nib (left) seems a touch springier and smoother than the one on a more recent Platinum #3776 Balance (right):
Writes very nicely with Sailor’s Waka Uguisu ink. According to the seller, this one was a dry writer. If you’ve a Platinum pen and have similar thoughts, you might want to try a Sailor ink before adjusting the nib.
On a side note: on page 115 of Nakazono’s “Fountain Pens of the World” (not to be confused with Lambrou’s far more comprehensive work), there’s a circa 1931 Dunhill-Namiki plunger-filler with an almost-identical shape and similar-looking celluloid with silver instead of brown rings. An inspiration for Platinum’s pen designers?
writing without rhythm 
Yes, that old feed is made of ebonite, and it works wonders!
Great pen.
BT