Function open_drop_at 

pub fn open_drop_at(
    identity: &PrivateIdentity,
    env: &DropEnvelope,
    manifest: &Manifest,
    chunks: &[Vec<u8>],
    now: u64,
    receipt_issuers: u32,
) -> Result<Vec<u8>>

receipt_issuers is the count of distinct stored witness/delivery receipts. Quorum seals are local policy, not a cryptographic time-lock.