Underworld Publishing released three adventures, fully "compatible with the most popular role-playing game." (aka Advanced Dungeons & Dragons 1st edition). Since all of TSR's product had a "letter code", their releases did as well.
The three releases are:
P1 - Sweet Wall of Death (UWD 1020)
P2 - Gladestrom (UWD 1022)
O1 - Harbinger of Darkness (UWD 1021)