A 'stamp bag' is a glassine bag folded up and taped, with a stamp on the front (i.e. a design or word or both cut out on a block, which is dipped in ink, and stamped on the bag). It's a form of branding, and name recognition. If word of mouth spreads that 'DOA' is good this week, everyone asks their dealers if they have any 'DOA' bags. Also how users communicate regional stuff.
Average powder content is somewhere around 100mg. Average Heroin (Diacetylmorphine) content varies. My estimate is 5mg-30mg Diamorphine per bag. Morphine, 6-Monoacetylmorphine, 3-Monoacetylmorphine, etc are also active opioids that are present in varying quantities. As are leftover alkaloids (Noscapine, Codeine, Thebaine, etc) and side products of the reaction (Acetylcodeine, etc). Some cuts are active (Procaine, Quinine, Lidocaine, Inositol, etc), while others are not (Lactose, Mannitol, etc).
It is impossible to judge purity by any means, be it taste, color, texture, weight, etc.