Yamaha model numbers work in that the last two digits tell you what pickup types the guitar has and the larger the preceding number is, the higher up the quality range you go.
So your 512 is in the 5 quality range with 1 humbucker and 2 single coils. The 721 referred to above will be in the higher 7 range with 2 humbuckers and 1 single coil. Middle digit is the number of humbuckers, last is the number of single coils.