Sun Roof

[will7262]

I see there is a photo, page 354 in Bristol Cars a Very British Story, of Blenheim 3's in the factory without their roofs. There are three roof supporting metal straps shown. The gap between the window head strap and that at the B post might be just big enough for a small Webasto.
This of course assumes that the roof of the 603 is constructed in te same manor. Given the nature of Bristol this is of course unlikley.

As for earlier models this form of roof construction might also be the same.

I will let you know the outcome.