I have seen stick on weights in lead, zinc, steel/iron (all usually marked appropriately; Pb, Zn, Fe), plastic and even a lead/"foam", kinda like a heavy, rubbery car door weather stripping (see this stuff mostly on VW's wheels) and probably some others I'm forgetting about.
The last 4, of course, being no good for anything bullet casting related.
What you have is either lead; usable for bullets or it isn't.
If you can make a mark with your thumbnail fairly easily, it's lead.
Clip on weights seem to have a lot more chemical variety in their makeup.
Again, the usual players: lead, plastic, zinc, steel/iron.
Of course the lead ones would be the only ones useful for (mainstream) bullet casters (although I see some folks experimenting with casting zinc bullets)
However, they're not going to be "pure" either.
You won't be able to dig your thumbnail into these (I use side cutters which will easily make a mark in "lead", extremely difficult to make a mark in zinc and impossible in Iron/Fe)
For whatever reason, clip on weights have different but excellent characteristics for (most/many) bullets compared to stick ons, appropriate %'s of things like antimony and arsenic (these chemicals allow for some "heat treatment" of cast bullets)
I'm sorry if this is outside the scope of your question but there seems to be some confusion.
Also, there have been books written on the subject so this is merely a "blurb" in comparison.
Hope it helps