>I'm wondering why it did not affect the VCX class compiled into COM, but did affect the PRG class compiled into COM...

If one of the projects is set to regenerate CLSIDs when you recompile, then that one will require that the MTS package be refreshed, because the package will hold a reference to the old CLSID.

For future reference, the involvement of MTS or similar technologies is relevant and important information needed to solve problems like this. We could have pegged this one on the first try if you had remembered to mention MTS... :-)