>>>Therefore the same instance cannot be safely accessed by more than one thread.
>
>Can't be accessed by more than one thread but can be accessed by more than one apartment.
>As long as there's only one thread per apartment, it's thread-safe.

But one thread per apartment is a single threaded apartment. Which is thread safe because, in effect, COM will queue the requests.
Aren't we discussing MTDLLs where there are multiple threads per apartment all accessing the same address space. In that situation any existing VFP code is unlikely to be threadsafe. Using Sys(2336) can mitigate the problems but it's very easy to miss situations where it is required. And if you are dealing with a multi-core environment as distinct from single processor thread switching it's even more problematic.....