With both 3 position switches in the centre off position, the Meathead stock values (10n & 22n) will be in circuit. When selections are made, the selected cap value is summed with the stock value to give a wide variety of stock up to Meathead Dark tones. You will have 10n, 22n or 43n for the input switch, and 22n, 69n or 122n for the output switch. Swap the caps to suit your own tastes if you like things a bit more or a bit less dark. It may be worth building the board and then breadboarding the cap selection part to make it ideal for you.
With the board only being 10 columns wide this is totally do-able in a 1590A. Let's see who's brave enough! :o)
After a conversation below it and doing a few calculations, it seems that the change of output cap for the Meathead Dark was irrelevant as the frequency cut by the standard 22n output cap was already lower than the lowest frequency on the guitar. With that in mind, this version may be more useful offering a post gain bass cut. In the centre position with just the 220p in circuit it will be a very lo-fi effect because much of the mids and all the bass will be cut. In the 22n position you will get a full bass output, and in the 1n position there will be some bass cut which may be useful for cleaning things up a bit.