The Ultra Rare Weapon bribe essentially gives you a Black Market weapon you don't own first. If you own all available Black Market weapons then it provides you with a new variant.
Ultra Weapon Bribes reward players with a new ranged Black Market weapon and two Reserve Cases when opened. If you already own a variant of all Black Market weapons, you’ll be rewarded with an additional weapon variant from your Ultra Weapon Bribe. Get out there, enjoy the new rewards, and have a safe and happy Fourth!
July 2nd Reddit Update
Ultra Weapon Bribes will reward players with a new ranged Black Market weapon and two Reserve Cases. If you already own a variant of all Black Market weapons, Ultra Weapon Bribes will award an additional weapon variant.
June 28th Reddit Update
Additionally, all players will be able to expand their access to more Black Market weapons as Ultra Weapon Bribes become available in the Contraband stream starting next week, and in future Operations.
June 25th Reddit Update
If people are encountering issues then I would need a video of it happening. Of what I'm seeing in PIK's comments are "my friend" or descriptions which seem to fit regular, non-Ultra, bribes.