Choosing which OS image to build from

After successfully building images with the 10.7.2 MAS Lion installESD, I've found a new iMac will not boot from the image - it gets the question-mark folder. I've used the same image on a new MacBook Air with no problem. Guessing that a change to 10.7.3 would be advisable, and cut down on the updates required post-install with a new image, I ran import.py on the new 10.7.3 MAS installer. I got the 11D50 file in InstalleFiles/InstallerDiscs as expected. However, with 2 files now in this folder, running the build chain that include the Vanilla catalog chooses the 10.7.2 image by default, rather than the more recent 10.7.3. The Vanilla catalog includes all the build numbers (I svn'd the latest build of InstaDMG). I'd like to keep the older installer until testing is finished, so what's the official way of making the base image choice - I haven't been able to spot that in the documentation. cheers, ---john---