The very earliest cars didn't even have the bolt in cross brace. Well after the bolt in brace came the welded in gusset plates and possibly(?) at the same time came a change in the steel specification for the chassis tubes and a fractionally thicker nominal wall thickness on them too. Sometime about the start of 2014 the bolt in brace was replaced by the welded triangulated tubular bracing. I don't know if the gusset plates were retained or deleted at that point.
