Hi Michael,
it was a special case
A manufacturer of aluminum castings in large numbers had first received STL files from me.
For reasons unknown to me, these were not enough for him.
Caused by the fact that the STL files were already checked and repaired with netfabb, I thought converting the STL file to STEP would be a good method. Obviously, this was the wrong way.
I now know that professional 3D service providers have good tools with which they can convert STEP files from MoI to STL.

We live and learn!
Thanks for your help

Bodo-Lutz