I have #20, 0276 is hull date. It seems to be evolving towards the 29.9 as it appears to have the 29.9 rig (longer boom, & 32' P), and a anchor platform/sprit.
The 29.9 post dates the 28. It seems to me that Bristol liked the hull, but wanted something more cruisy - wheel, no bridge deck, and no IOR rig and a centerboard / shoal draft. The move worked as many more 29.9 were built and it seems the production lasted almost until they went under?
If anyone has more info, please share - I am new to the Bristol world.