Read attributes CircuitDiagramReference and SingleLineReference in plugin

Have a nice day,
I would need the following items in the C# plugin in the material list:
AttributeId.CircuitDiagramReference (ID = 9)
AttributeId.SingleLineReference (ID = 81)

I would like to select them from the attributes and read their value from the filtered material collection, or from the selected node/ObjectItem

see code:
string bomItemCircuitDiagramReference = itemMat.Attributes.GetAttributeValue(AttributeId.CircuitDiagramReference);
strin bomItemOverviewDiagramReference = itemMat.Attributes.GetAttributeValue(AttributeId.SingleLineReference);

I can read other attributes in the plugin like this, but not these.
In the BOM working list, using these attributes works, but I can't read them in the plugin.

Thanks in advance for any experience or idea.

Thanks MK