I took a while to reply to this. This C139 may as well be a custom multi-protocol/data link controller possibly with DMA. I can confirm it's a ring style protocol as the style for connecting Namco cabinets (at least from the manual) is similar to connecting Sega cabinets. Both have a send/receive port.
I like how my Motocross Go machines have two different USB ports, A and B, making the send/receive ports a bit easier to tell.