There are already separate topics in General with this information, so it does no harm (and it's more convenient) to gather it all in one place.
RCS stamps
Five are rewards for completing optional levels:
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×(1) Gentryii Grounds (requires the first Rooted Hub keycard)
(2) On Dawdling River
(3) Braxian Below
(4) Hardest Way (requires completing Harder Way)
(5) Tar Recycling Annex (reached through Forbidden Underpass)
The other three are found in secret rooms:
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×(1) The Rooted Hub
(2) Icy Underworld
(3) Seeding Grounds
Rooted Hub keycardsClick here to view the secret text
×(1) Secret room in Training Hall
(2) Awarded for completing Frozen Wood
Arky roomsClick here to view the secret text
×(1) Forbidden Underpass
(2, 3, 4) Security Gates
(5) Forgotten Shrine
(6) The Scorching Path
(7) Ore Refinery
(8) Explosives Storage
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50th Skywatcher