A search has revealed 3 folding cameras that allow for a 600mm lens. One may not want to but I wonder if such a camera ,that can entertain a massive lens at full extension , may also be more robust and firmer at intermediate focusing distances.

Layton L-45A 65-617
Lotus rapid 85-600
Canham Traditional 76-660

Can anyone shed any light on these cameras, apart from their prices.