Confirmed
Switchblade
A switchblade lies next to the Morgan revolvers in Rockstar's picture of the Ultimate Edition weapons.
Notes
Rockstar's picture of the Hawk & Little Morgan revolvers on its editions page shows a switchblade on a stack of cash beside them, and Rockstar's own caption for the picture names it.
Whether it is a melee weapon you can carry has not been said.
Sources
Grand Theft Auto VI editions: Ultimate Edition and pre-order bonus (opens in a new tab)
Pictures the switchblade with the revolvers and names it in the image caption.
All of the Grand Theft Auto VI weapons seen so far (opens in a new tab)
Spots the switchblade and expects it to be a melee weapon.
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