A recent discovery found inside AMD's latest Adrenalin driver suggests that AMD is preparing to launch a rebrand of its existing RX 550X graphics card as the RX 640. Since all next-generation Navi cards are expected to follow the RX 3000 branding, it's possible that AMD is wanting to better distinguish Navi from Vega by restricting the RX 3000 moniker for Navi cards only. This type of brand distinction has been done before, the most recent example being Nvidia's RTX 20 series versus the GTX 16 series that lack ray tracing and tensor cores.
Actual details about this card are unknown, including if the card will use the new 7nm process node or what the clocks and power target will be. The current RX 550X is a sub-$100 graphics card based on AMD's long standing Graphics Core Next (GCN) 4.0 architecture and is currently produced on the 14nm process and features 512 shaders, 32 TMUs and 16 ROPs with a power target of just 50 watts.
A rebrand of this card would be aimed at the entry level gamer or esports market. AMD has focused on the entry level and mainstream gamer first when releasing new architectures and then addressed the high-end and enthusiast segments later as the volume of cards sold is considerably lower at the high-end. At the very least, Navi seems to be just around the corner (finally).