Would This Be Classified As A Cinderella, Or An Etiquette, Or What?

I recently acquired this piece. Am totally confused as to what to think about it.

left (yellow) flag: PTT Serbia for children.
right (red) flag: 150 years of post. (this really confuses me, because it would date it as 2016, but I know that it is older than that.)
top legend: postage exempt.
lower legend: JUDEF/Yugoslavia childrens' fund.

it is postally cancelled, but being off piece, I have no idea if it was added with regular postage, or in lieu of regular postage.

I don't even know if it is officially issued by PTT Serbia or PTT Yugoslavia, or just a private entity.



thoughts? comments?

Opinion:
Yugoslav charity seal : All Funds : Yugoslav Children's fund.

(I'd have JUDEF in the image text string for easy recall / search text)

That's how I would locate it in my database / Album.