Once you saved your game (beat the game first!) mechs that appear on days that are not divisible except by itself (i.e., days that are prime number days, like 2, 3, 5, 7, 11, etc. - break out your math book) are super-powerful.

For example, between days 20 to 30, the Wild Mechanics will have a used SV with quad legs, heavy core, and one arm cannon at a cost of 1,500,000.