I remember seeing them in many of the old-style mechanical pencils, in fact, I have one at home I will look at tonight.

Interestingly enough, my Mr Peanut (no, I mean the pencil ;) appears to only have the eraser stuffed in there, I don't see the typical metal eraser holder thingie that you are talking about. Hmmmm...so maybe it's no biggie (historical authenticity-wise speaking) to bust mine out. Apparently someone just stuffed a non-standard eraser in there.

If I remember correctly, the eraser-holder-thingie (of course, this is the technical name for the eraser doo-hickie) with the curled edge is not solid all the way around...seems like if I remember correctly, it has open sides so that by pushing it down into the pencil, it squeezes the sides of the eraser in order to keep it in (wow, this whole post seems mildly obscene ;) )