OCX migration from D2 to D3

Why does Delphi want to add _TLB to the end of the OCX interface file
name?  I can find no documentation, and Delphi can't compile the app
because it can't find that file once it replaces the real name of the
OCX with the realname+_TLB string.  

This app was happily running in D2, but I needed to upgrade to
Client/Server, so now I'm in D3.  After trashing my component palette
trying to install the OCX, I finally got it installed (and reinstalled
my component palette), but I am just starting the migration and am
getting a funny feeling . . .

What's going on here?

Jeff Jones