No, I'm just going off memory. As to your PokeDex notion, I have little to rebuff. However, I must note that, in Pokemon Origins, something very closely based off the games, Blue's Pokedex had no data of Mewtwo, but I would say that this is nothing much to base an argument on. Though, Deoxys is hardly known at all, so it is a little ludicrous to think that the Pokedex has data off it, as well. Same with other legendaries. Also, if Magcargo was truly as hot as the sun, Earth's atmosphere would literally burn away from the planet, so I would guess that there is an element of "guesstimation". Though, I must say that no one ever refers to Mewtwo by name aside from its creator -- they only refer to it as a "horribly strong Pokemon" or "monster". It's pretty clear that no one knows about Mewtwo aside from the obvious creators.
To the point that the Pokemon Mansion was sacked recently, I must refute this claim. First and foremostly, electrical wiring doesn't degrade in a month of disuse, as plenty of things can still fire up after years of disuse and, of course, a new battery. But seeing as it would be on the Kanto electrical grid, I would doubt that it would need such things. Scavengers are also things, and I would doubt that as soon as the place was ransacked the entirety of the populace came to loot it. Finally, I wish to bring your attention to the journals. The remark about how Mewtwo was born on February 6th, and how it escaped on September the 1st. If we go by when the game was released and use that as the average point in time of when people played it, thus giving us a point in time we can wager that Red and Blue take place, we come to the Japanese release of February 27th. Therefore, Mewtwo would have to be over a year old, and would have had to have escaped at least five months prior to Red's journey. Even if you refute this claim, you cannot deny that Mewtwo is still at least eight months old in Red and Blue.
Plus, we have the case of Mr. Fuji -- a kind,
old, man. I would doubt that he decided to be a kind, old hermit after brazenly playing God with Mewtwo merely five months beforehand. But that rides on the assumption that Mr. Fuji is the same as the "-ji" who messed with Mew. This could be someone else entirely, but it's widely assumed that he is one in the same. Even so, it isn't like Fuji just came to Lavender Town, set up shop and gained the love and respect of the locals in just a few months. Stuff like that takes time. Especially aging.
To your ten year claim (even if Bulbapedia is once again proven wrong), again, I would doubt that there would still be any hype left to put up posters. But perhaps that is just another reason why Team Rocket should move their switch to get into their secret base.
Even so, the fact that Fuji is old and wise is no doubt evident of his past -- something that would take years to sink in. I would wager that it has been quite some time since Mewtwo was created.
I'd doubt that there would still be publicity blurbs about it after at least eight months. Porygon is the first
known man-made Pokemon. "Known", being the keyword. You yourself mentioned Golett and Golurk being ancient, man-made Pokemon, and no one can really be sure about Sigilyph, either. Ditto (ha!) with Magiana. I think it's fairly evident that Porygon isn't the first man-made Pokemon.