The door once stood at 10050 Cielo Drive, but had quite the journey following the grisly Manson murders in 1969, according to Christopher Moore ... the door's current owner, who's selling it through Julien's Auctions.

Trent recorded his group's "Downward Spiral" album in the house while renting it in 1992 ... but Christopher tells us the singer took the door with him when he moved out the next year before the place was razed.

And, as we reported, "Full House" creator Jeff Franklin's mansion sits on the land today -- and that's been re-listed multiple times, first going on the market for $85 mil but now it's just under $55M.

Christopher says Trent went on to use the door as the entry to his New Orleans recording space -- but he relocated the studio to L.A. in 2004, and put the NOLA building up for sale.

We're told a doctor scooped up the property in 2012, but took the front door out of the trash when he found out its historical significance.

From there, Chris says he bought it off the doc in 2017 and was planning on installing it into an old church he owned, but the historical district said no dice ... so, instead, he put it up on the auction block.

The piece was expected to fetch anywhere from $2,000 to $4,000, but it's skyrocketed past $22k with just 7 bids! FYI, the auction doesn't close until September ... so we're guessing it's gonna keep climbing.