The logic for Toyota part numbers is like this (generally, with some exceptions):
1. First five digits is the component or system area that you are dealing with so in your example 31210 is a part number category that generally corresponds to the first 5 digits of a ten digit Toyota part number (PNC as we call it). These correspond most of the time, but there are sometimes where the PNC number is not the same as the first 5 digits of the ten digit number.
2. 2nd set of 5 numbers indicates the vehicle usage and indicates generational aspects of the part number (engineering changes, supplier changes, etc) (ie: -6, -61, -66, -90, -17, -54 indicate Land Cruisers). You sometimes also have applicability of numbers across vehicles (in the case of tools and oil filter etc.).
3. Sometimes you have a -77, -83, -84, -C0, etc placed after the ten digit number and those suffixes indicate things like "remanufactured, locally sourced, or color codes.
So a good example of a Land Cruiser number: 16110-61081-77: (water pump for a late 2F): Notice, for example, that the PNC is actually "16100" but the actual part for the water pump is "16110".
Hope that helps.
