Originally posted by Mr. X:

IDS:
BossStone_Bonemass
BossStone_DragonQueen
BossStone_Eikthyr
BossStone_TheElder
BossStone_Yagluth

see: https://valheim.fandom.com/wiki/Item_IDs#Other

Those appear to be the codes for spawning the trophy stones at world spawn for each boss, not the actual boss summoning altars. Sorry for the confusion, I should have been clearer. Would you happen to know what the codes are for the boss summoning altars?