Dan, The photo without any bracing looks to be a very early car, maybe even pre-production - it also looks to have the original Suplex shocks and the headlamp brackets are welded on. My one had sat effectively untouched for about a month before the crack was found so while you are probably right that it had been cracked for some time the rust marks could be misleading. Don't fancy persuading (or paying!) someone to model the stresses and the propagation rates.
