Manhattan prosecutors have quietly offered a no-jail, misdemeanor deal to the retired young Marine who’d been facing a mandatory 3 1/2-year prison sentence after trying to check his Indiana-registered handgun at the Empire State Building, court papers reveal.

But the former tow gunner, Ryan Jerome — who has garnered letter-writing support from hundreds of current and former Marines from across the country — is saying no thanks, and continues to ask that the case be dropped altogether.

The DA’s conciliatory move — and the fact that it’s been on the table for two months — was revealed in a letter filed in Manhattan Supreme Court.

Throughout the two months, Jerome has argued through his lawyer that he does not deserve any criminal record — felony or misdemeanor.

“We continue to believe the case should be dismissed outright,” said Jerome’s lawyer, Mark Bederow, declining to discuss the negotiations.

Jerome, 28, is a West Bend-based jeweler, and had brought his legally-registered .45-cal Ruger to New York on a business and pleasure trip because he was transporting $15,000 in gold to a Long Island refinery, he argues.

He was arrested during a visit to the Empire State Building, when he alerted authorities to the gun after seeing a sign that said all weapons must be checked.

It is a felony — carrying a 3 1/2-year mandatory minimum sentence — to posses any gun in New York, even one legally registered elsewhere, unless it is registered in New York. Prosecutors typically use discretion in charging folks caught with guns, sometimes offering pleas to misdemeanors as in Jerome’s case, but almost never dismissing the charges outright.

Before coming to New York, Jerome had looked at a website that described which states he could legally bring his gun to — but was confused by how the columns were formatted on his screen and sincerely believed that his Indiana license was sufficient here, his lawyer has argued.

“The New York District Attorney’s Office is offering to allow your client to plead guilty to a single Class “A” misdemeanor in full satisfaction of all charges,” said the letter, addressed to Jerome’s lawyer from the prosecutor heading the case, Joseph Davis.

“This plea deal is made available to your client with the understanding that should he decide to accept said offer, he would be sentenced to pay a fine of $1,000.00 and required to perform 10 days of community service at a place both parties could agree was appropriate,” the prosecutor’s letter read.

Jerome is due back in court March 20 to learn if he has been indicted.