While "PCG-31311M" is the regulatory chassis number found on the bottom, Sony uses a separate model name for driver support (e.g., VPCYB15AG or VPCYB35AG ).
Sony’s official global support site ( https://www.sony.com/electronics/support ) no longer lists the PCG-31311M for direct download. However, there are several reliable alternative sources: Sony Vaio Pcg-31311m Drivers Windows 7