Why slack rigging

Till the mast crumples like spaghetti!

Both my Fireflys were fitted with rotating masts, the fastest setup was to have the forestay slack enough that the mainsheet could be block to block upwind. For downwind, there was a jib tensioner to pull the rig forward and tighter the jib luff.
If one forgot to release this upwind, the boat was significantly slower.

As Rupert states, the Reynold masts needed tight diamondstays - the section as too thin to be self supporting downwind (remember that the mast rotated).
The later Proctor masts were more robust, nowadays, the mast is fixed with spreadrs, so it's a different setup.

With regard to the Solo,one of the effects of slack rigging is that the mast can sag a bit to leeward, tending to open the leech upwind. With the fully battened sail, I suspect it is too easy to hook the leech.

Colin

Ive not seen a normal D on a Solo for years. I would imagine the gold masts are Ds and Cs not seen a B but i bet they were used too.