The type certificate data sheet for the Stearman can be found here:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/1b66a9575ec373858525673000457610/$FILE/A-743.pdf
Searching "solo" reveals limitations which are not entirely straight forward, and some nearly self contradictory. So maybe, TCT actually read the TCDS, and could not figure out how to fly it solo within the C of G range, so gave up, and just carried ballast ('may as well carry experienced ballast!).