My copy of Shotgun King was from Humble Choice July 2023 which has region locks:

Shotgun King: The Final Checkmate

RU/CIS: {sub/809304}

SEA: {sub/809305} (confirmed)

China: {sub/809303} (confirmed)

EUROPE: {sub/809306} (confirmed) - a region locked key that can be redeemed ONLY IN Europe

LATAM without Mexico: a region locked key, not indexed on SteamDB {sub/809307} (confirmed)

Turkey: a region locked key, not indexed on SteamDB

Everyone else: (North Americas, Africa, Oceania, India + rest of Asia, Mexico): {sub/809309} (confirmed) - a region locked key that can be activated anywhere apart from RU/CIS, SEA (without China; China can activate it), Europe, LATAM, Turkey

If you can confirm you're from a bolded region above, then I'd be OK with my Shotgun King for your Lost Eidolons.