The first 4mm arm to break was a near full speed impact with a wire fence hitting arm first. The arm got soft and cracked as a result of the 30-40mph impact. When using 3mm arms one must keep the flights in check to avoid unnecessary damage. The 4mm arms do seem to represent the sweet spot.
The whole thing is just a bunch of math, spot us, we got 1536.
2 x 2 x 2 x 4 x 2 x 2 = 128 choices
If dual bottom plates there are 4 immediate choices for thickness (1mm+2mm or 1.5mm+1.5mm). Number of dual plate combinations 128 * 2 = 256. If single plate 3 choices for thickness (2mm, 2.5mm, and 3mm) for the base and 2 for the top (1.5mm and 2mm). As a result the number of single plate combinations (3 * 2) * 256 = 1536.