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Thunder Force: Gold Pack 2 (サンダーフォースゴールドパック2) is a compilation of shoot-'em-up games from the Thunder Force series, and was released exclusively for the Sega Saturn by Technosoft in 1996. It is the immediate followup to Thunder Force: Gold Pack 1, released just a few months after. Gold Pack 2 itself was not released outside of Japan, though the games within it have seen various export releases.
The port of AC appears to have scoring changes. Needs more research.
The port of IV has mostly quality of life changes:
There is no slowdown present.
Sound effects seem clearer, but lower in volume.
The "Styx" ship from Thunder Force III can be selected after clearing all the stages on 1 credit (no continues) and entering a special code given after the ending. At the main Gold Pack 2 game select screen, highlight Thunder Force IV, then hold + and press START . Continue this on all menus afterward until the game starts. The Styx ship as all weapons and sound effects from Thunder Force III; even the game's normal font changes to match!
Additionally, both games save scores to the Saturn's memory.
This version of Thunder Force IV was ported (with additional changes) to the Nintendo Switch as part of a new Sega Ages series. A port of this version of AC has also been announced, though it will likely also include the original arcade version.