As I understand a drafting machine, the idea is to keep the edges of the square at the same angle wherever it is moved over the surface of the drafting table. To accomplish this, it is NOT necessary to have four equal length arms, but two sets of two equal length arms. That should simplify it somewhat.
You do not have to get these already drilled to get what you need. Simply cut two lengths of metal, I would use aluminum but what you want, and clamp them firmly together. Then drill the holes in each end without loosening the clamps. The two lengths will have those holes exactly the same distance apart.
The brackets (what I believe you are calling blocks) used on each end of these arms will also need to have a pair of holes that are the same distance apart. Even though they may be of different shapes, they can also be clamped together to drill this pair of holes.
This will probably produce a better match in the distances between the holes than any pre-drilled stock you can buy.
If I were doing this, I would use a slightly small drill to drill the holes and then use a reamer to finish the holes to size. Then you can use some precision drill rod or perhaps some shoulder bolts for the pivots to get the best precision.
The shoulder screws have a shank section which, like precision drill rod is very precise in diameter.
Make it fit.
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