Because of the timber frame used to hold the roof onto the cab 12 (and I suspect cab 11) we see a number of different solutions to the problem.
What happens is as the wood shrinks and softens with age the cab separates at the windscreen pillars. I've seen this even in wartime images, so it was definitely a design problem.
Fixes range from long braces to this relatively elegant (but ultimately failed) solution.
What examples of running repairs have you seen?