According to VideoCardz, ASRock has developed a motherboard expansion card with a built-in Promontory21 chipset, which can be installed on the B650 motherboard to expand it to X670.
Image Source Level1Techs
According to reports, Level1Techs received a B650 LiveMixer motherboard kit that includes this tentatively available motherboard expansion card.
AMD X670 motherboards use two Promontory21 chipsets, while the B650 series has only one. Therefore, ASRock wants to expand the B650 to X670-like specifications by adding the chipset directly to the expansion card.
This expansion card with its own chipset has two PCIe x4 NVME M.2 slots, three USB Type-A, one USB Type-C, two SATA connectors and one 10G network port.
Note that this expansion card is only available for some ASRock motherboards and requires a special interface connection.