Hello grob,
The A cat can be strickerless because the loads on the main beam are so small. The loads come from the max righting moment and the A cat is a one person boat trapezing on a 7.5ft beam and the platform itself is very light weight, 165pounds, so the loads are small, about half, relative to a two person boat for example.
The Supercat product line was the first boat to go strickerless, 1980. The SC15 and SC17 are both strickerless two person boats with the aluminum main beam being 4.5inches in diameter.
Good Sailing,
Bill