That would be a Carleton 14 and yes, the bottom has a layer of end-grain balsa (comes as a sheet, 3/16"-1/4" thick blocks temporarily held together with a mesh-like layer on one side). It adds stiffness to the bottom layup with less weight than solid glass, but also a bit of liability in that you don't want to puncture it and get water soaking in there. The designs are similar to some of their old wooden boats, though usually a bit more flat-bottomed. By that time, I'm pretty sure they just made a normal plug and pulled a mold off of it. There isn't much to be gained by taking molds off of old boats or forms when you can start from scratch and get the new one more fair with less work.