I have referred to bolts/studs that came with my kit. Those came in different lengths, with male threads on both ends. When I tried a few in the T-nuts, some went all the way through, although then seizing at some point.?
My 1-2-3 blocks were a separate purchase and not part of the clamping kit. As you stated, they have two kinds of holes. Most of the holes are too small for a 3/8, but there are 5 holes threaded for 3/8-20. Those 5 holes are all on the biggest face of the block, located as a centered "5" face of a gambling die.
