The stand is probably a TE-E or was part of a package with boom, the TEB-E. There are no parts listed for it on the Atlas site and the model has been discontinued. If you still have the booms you might find the wayward piece threaded in the mount.
A call to Atlas might be useful, though, as they may have some parts in a bin somewhere. The current tripod stand has a threaded 5/8" tube rather than an insert.
If you can't find parts I suggest tossing them in the recycle bin and refer you to a thread over at PSW:
Cheap durable mic stands