[rotorwrench]

A lot of the info I have found doesn't indicate that the three pieces were attached to each other. This may be the case but I'm not the one to be able to say for certain. The top bows were likely mortise & tenon jointed together in the corners but the tack strip may have not had a need to be. Some wood was structural but some may only be for trim purposes and mounting structure. The top center bow is a structural member for sure. A lot depends on how well the body lip where the tack strip mounts was formed. There are a few Cabriolet guys that frequent here. Hopefully one will respond.

Mortise and tenon joints can be difficult to fabricate and pining them with dowels further complicates that. The dowel bores and dowels require some accuracy in tooling to get good fit but ultimately, it's the glue that holds it all together that is the most important part. All the pieces just provide more area for a good solid glue joint.