like it's hanging off the PCI bus. The vendor/device matches to an Exar Corp. XR17C152, which is given a base_baud of 921600 in drivers/serial/8250_pci.c.
According to the chip specs, 921600 is correct with the default crystal and control register values. However, other crystals can be used and there's also a control register bit to enable a prescalar which cuts the baud rate by a factor of four. I suspect that one of these has been done on our boards.
Probably. Did your vendor have the sense to set subvendor/subdevice
identifiers that can be used to identify that specific unit ?