Parker Duofold - Made in England

[MarcoA64]

Good evening folks!! hope you are all doing well.
I have had this pen for decades, all I know is that it is a Duofold. Big deal, it is written on the pen barrel. I bought it at a flea market in London many years ago, I was there very early with a friend of mine around 6 AM. He bought his things and I bumped into this pen. It has always worked, from the first day. I just filled it again today and I have noticed that the sac is a bit gooey.
It fills and writes very well.
What I don't know is the year it was produced by Parker and what its real model name is.
Is it possible to change the sac with a new one? Thank you for you support.

Duofold 1.jpg

Duofold 2.jpg

Marco