AT903 VFD modbus register map

Hi all, 

After some success with modbus comms using pkuart and hm2_modbus (thanks to PCW for all the support) I'm minded to try talking to the VFD with modbus as well.  However the vfd is (cheap, amazon-sourced) model AT903 of the AT900 series, maker unknown, and although the manuals I've found show how to set it up for modbus comms they do not include any kind of register map. There's a parameter for '0=standard modbus 1=nonstandard modbus' and a number of other params related to serial comms, but that's it. 

Has anyone successfully talked to one of these over serial, or knows who make them, or has the register map? 


Andy