Erik and Lyle Menendez are experiencing newfound fame behind bars thanks to the popular Netflix series about the gruesome murders of their parents … and there’s a weird request that keeps coming up in the fan mail that’s been pouring in, TMZ has learned.

A source with knowledge at Richard J. Donovan Correctional Facility in San Diego tells TMZ … the infamous brothers have seen a “huge” increase in mail from fans (and, of course, some haters) thanks to wild interest in Netflix’s “Monsters: The Lyle and Erik Menendez Story.”

Perhaps the most interesting item the brothers are being sent … 1990-91 Hoops Mark Jackson NBA cards — the brothers are on the card, snapped sitting in the Knicks crowd, after the murders, before their arrests — and people want ’em autographed.

The brothers … who spent years incarcerated apart before being reunited in 2018 at the San Diego prison … were recently paid a visit by Kim Kardashian, among other high-profile names, in her quest for prison reform. Kim has since said the brothers should be set free.

As we previously reported … the brothers are again making national headlines after L.A. County D.A. George Gascón announced prosecutors are reviewing new evidence … and he will decide if the brothers should get a new trial, or if prosecutors should go for a new sentencing — presumably manslaughter — which could set them free.