All eyes are on a never-before-seen side of “Ol’ Blue Eyes.”

A 1964 letter Frank Sinatra penned on the kidnapping of his son is being offered at Bonhams’ Unplugged & Unforgettable Music Auction , which starts Tuesday.

The six-page letter to a prison chaplain, regarding the infamous 1963 kidnapping of then-19-year-old Frank Jr., is estimated to fetch up to $30,000.

“The raw emotion portrayed in this letter reflects a man speaking openly, with less concern for the confines of public scrutiny. It’s this rare vulnerability that makes us especially excited to offer a glimpse into another side of a legend like Frank Sinatra,” Katie Allen, associate specialist for Bonhams pop culture in Los Angeles, told The Post.

In 1963, two amateur criminals, Barry Keenan and Joe Amsler, kidnapped Fra

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