Starting with the Mark I, the Spit generally used round-head rivets along seems where major panels attached to the major longerons and major bulkheads. In other words, along the outer edges of the panel. Flat head (flush) rivets were then used along the intermediate stringers intermediate bulkheads. I believe this practice was followed from the Mark I through the Mark V, as well as through many of the Mark IXs, since the early Mark IXs began as Mark Vs and were changed over to Mark IX configuration during assembly or in later retrofit. Later Mark IXs, and subsequent marks, used flush rivets throughout due to changes in fuselage skin thickness and improved manufacturing techniques.

Hope this helps.

Charles