EXCLUSIVE

The multi-million dollar guitar famously owned by Kurt Cobain -- the last one he played before killing himself -- is at the center of Frances Bean Cobain's nasty divorce.

Frances' estranged hubby, Isaiah Silva, claims she gave him Kurt's Martin D-18E as a wedding present. It's the same instrument the Nirvana frontman famously strummed during his epic "MTV Unplugged" performance.

The acoustic-electric is something of a holy grail for Nirvana fans. For starters, only about 300 were made before Martin discontinued the model. Kurt, who was left-handed, had the bridge and nut replaced so he could play it upside down.

Frances denies giving Isaiah the guitar, and wants it back, with good reason. It was once insured for $1 million and experts now think it's worth several million bucks.

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