Re : RS422 module from Aliexpress
Web searching on the module part number finds assorted seller pages claiming that TVS diodes are provided.

The mystery components are in a sensible position for such a role.

An SMD package such as DO214 is suitable for such a component and has the general appearance of those on the board, though the exact physical sizing isn't immediately easy to guess. 

Undocumented modules are always a gamble.  On the plus side, simple ones where you can visually follow the traces and readily re-work the external components can give you some flexibility to quickly get something going.  On the minus, you have to take the time to understand, and you are sourcing the central chip from the grey market...