1-2-3 blocks (aka setup blocks) are a highly configurable drill press and milling fixture. They are most useful when you need a straight edge or a fixed support surface. Normal 123 blocks are made of hardened steel ground perfectly square and flat. This is intended to be a cost effective & disposable version. (Steel blocks on amazon for comparison: http://amzn.to/28PFOgM)
The block dimensions are 1” x 2” x 3”. The holes (5/16") are used to fasten the blocks to each other or to your table.
Setup blocks go along nicely with step blocks (clamp set): http://www.thingiverse.com/thing:50512
Print with rectilinear infill at 30% minimum. The clamp uses a 5/16 nut pressed into place.
I recommend purchasing a 24" length of 5/16" threaded rod and cutting it into (2)-5" pieces, and (4)-3" pieces.