# Pastebin l8Ii1iTg
missing_mbids = set(your-list-of-mbids)
earliest_scrobble = {}
for record in dataset:
    mbid = record['mbid']
    if mbid in missing_mbids:
        if mbid in earliest_scrobble and record['listened_at'] < earliest_scrobble[mbid]:
            earliest_scrobble[mbid] = record['listened_at']
        elif mbid not in earliest_scrobble:
            earliest_scrobble[mbid] = record['listened_at']