KISS 'Alive!' Handwritten Letters Up For Auction, Could Fetch $150k

Some KISS memorabilia is hitting the auction block and expected to fetch a pretty penny -- upwards of hundreds of thousands, in fact -- and this stuff isn't even musical, per se.

Instead of music, it's letters ... handwritten by each OG band member, for their fourth album "Alive!" ... and featured in the gatefold of the groundbreaking live album. They're up for grabs through GottaHaveRockandRoll.com, which is expecting these personal notes to reach bids between $50k to $150k.

Like we said, the KISS rockers put pen to paper ahead of the release of "Alive!" ... writing separate pieces of correspondence to their fan base in pretty descriptive and unique terms.

There are notes from Gene Simmons, Paul Stanley, Ace Frehley and Peter Criss, respectively.

The content is quite different in each letter -- Gene's talking about punishing/pleasuring listeners, Ace is on another planet, Pete's feelin' feline-ish and Paul's getting into voyeurism. The letters match each rocker's stage persona ... The Demon, The Spaceman, The Catman and The Starchild.

The soon-to-be owner will get the 4 pieces of paper -- inheriting them from a lucky then-15-year-old, who initially copped them in 1976 -- complete with the glue an art department applied way back then as well.

They're all certified legitimate through multiple authentication agencies ... so we're talking one of a kind.

The current highest bid is $20k and the auction ends April 14 ... so there's still plenty of time to rock and roll all nite, if you're into it.