Proper Japanese Title: 機動戦士ガンダム サンダーボルト 外伝
Originally serialized in eBigComic4.
Spin-off of Kidō Senshi Gundam: Thunderbolt.
— Description from Comic Vine community (comicvine.gamespot.com · https://comicvine.gamespot.com/kido-senshi-gundam-thunderbolt-gaiden/4050-161808/).
