Be careful when you buy them as lots of the inexpensive ones seem to have interesting hole sizes. The tapped holes are fine but the not-tapped holes are the minor diameter of the tapped holes. Therefore a bolt that will screw into the threaded hole will not pass thru the non-tapped holes making 'bolting them together' almost impossible.

I spend an afternoon calling vendors asking if they had 1-2-3 blocks with the non-tapped holes drilled to the clearance size of the tapped holes, not the minor thread size. Most of the vendors were good enough to actually check and were suprised the untapped holes were too small. I finally found what I was looking for at MicroMark. I ordered several sets an they do work the way you would think they should. Alas others who have called and\or ordered since them have reported that what they have received have the minor diameter size holes and are therefore not nearly as useful.