The enclave facilities in 76 are the oldest in-text source I can find related to the enclave creating deathclaws, but I swear to god I remember one of the 3d games having a loading blurb that at least confirmed they were created pre-war. Though personally I wouldn’t count the manuals completely out, they’re the whole reason why pip-boys are wrist mounted now.
I also vividly recall having a conversation with a guy who claimed deathclaws didn’t have horns until the master altered them, but I can’t find a source for that either.
I've heard the thing about the loading screen thing a few times but I've never seen it and I can't find it on any site that catalogues the load screens for the 3d games. I know the Master says he refined the deathclaws with FEV but he's not very specific on that point. The Bible is also wishy washy about specifics on what types of mutants are FEV mutants.
There was always an air of mystery about these creatures origins in the beginning. I know from watching Tim Cain's videos he deliberately didn't want the Deathclaw's to have a cut and dry origin.
I don't disagree with any sort of retroactive canonicity of some documents I'm just personally not interested in them when it comes to personal interpretations of the text. Especially when so much of it has been just as easily ignored/abandoned in favor of modern creatives interpretations of the setting.
