Green’s Dictionary of Slang

peg n.7

[ety. unknown; ? link to opium jargon or peg n.4 ]

(drugs) heroin; a capsule of heroin.

[US](con. 1948) G. Mandel Flee the Angry Strangers 303: ‘Here’s a few caps,’ [he] dropped six pegs of Horse cut with sugar into her hand.
[US]ONDCP Street Terms 16: Peg — Heroin.