Hi Malcolm,
It looks like IronCAD does not support importing & exporting any Custom properties with step file. Mapping & remapping STEP file's properties requires data in the DATA section of the .stp file. The attached image shows what you see in the Property window. In this order, Part Name, Part Number, Description. You can edit this texts with Notepad.
I tried deleting and adding some Custom Properties and then exporting and testing them, but nothing was added to the DATA Section.
112 GREEN PART.stp112 GREEN PART naked.stp112 GREEN PART extra.stp
These files are physically the same things, just with different file names.
So, in order for IronCAD to save and load Custom Properties into a Step file, it seems that we need the mapping and remapping tool.
Kim