Assistance Needed For Identifying A Swiss Streubel Stamp

I have a problem for identifying a Swiss stamp type Streubel 20c with green silk thread.
The reason is very simple: check the attached scan and you will immediately understand.

I placed it among other 20c orange and near the 5c brown. So you can see the difference of colours.

There are only 3 types of Streubel 20c : Zumstein B, F, G; all with green silk threads.
I have checked carefully the varieties of the 20c in the special catalogue of Zumstein and Michel; I have also checked in Yvert and Scott.
Nothing !
Nowhere it is mentioned such a brown; it is always yellow-orange till orange-yellow.
Has one of you already seen such a colour for the Streubel 20c ?
If yes, how is it classified?
Thanks for your help.

Any time I see an orange stamp that has darkened, I start out assuming that sulphurization is at work.

Yours is quite dark, and quite uniform, but that is the only guess I have.

Thanks Cjd.
I don't know anything about sulphurization, but sometimes yellow stamps are becoming a little bit darker. But not dark Brown !
And as you said: it is uniformly Brown and not stains.
having the stamp in my hand, I can confirm that it looks loke a Brown color was used... but I am waiting the opinions of specilists in Swiss stamps.

I saw nothing in SBK, Michel or Scott Classic.

Oranges can definitely make it all the way to brown when sulfur and metals in the inks mix. Check out the photos at this old thread:

https://goscf.com/t/6683

I don't dare to plunge the stamp in hydrogen peroxid because of the possible bleaching if sulphurization is not the culprit!

I don't blame you for that. I was just using the link to show the stark differences possible with orange inks. I think it is one possible explanation for your stamp. Maybe.

re.

" I think it is one possible explanation for your stamp. "

make that definite explanation,

Pagoda