If the item was created in 2006, it will have the 2006 version within it.
I understand tht while the item downloads, it will not show in Surveyor - nor in the program.
The same hapens with items created in 2006 SP1 - they don't show in TRS 2006 (without SP1).
It's quite possible to edit the item - provided it doesn't use the advanced features, then it should behave normally.
For example, a tree created in 2006 is most likely to be editable and useable in 2004. A locmotive that uses the full 2006 SP1 features may be edited to load into 2004, but will not work correctly.
HTH,
Colin