The difference: 8051 - 8031 = 20

The 8031 originally didn't use any internal program space. Therefore it requires a lot of external components like EPROM etc. This made it more like the Z-80 and 8080 of that era. It may have once also referred to factory burned ROM which is was a headache thankfully no longer necessary.

The 8051 was a true microcontroller wherein the single chip held everything needed to process your code and the chip's pins were used for I/O, power and clock.

8051 is all you need. 8031 is archane.

aka j