Hi,
I think you are looking at Type - Properties - Model and then selecting the iProp Part Number. If you will notice this is a "model" property. As you are aware that an iam is not a model but a collection of models. So when you place the sketch symbol it is going to give you the Part Number of which ever part in the assembly that you pick. There does not appear to be a direct way to make a sketch symbol grab the Part Number iProp from an iam. I have an work around for it. Change your local balloon style to use the Part Number iProp. This will allow you to make reference to the iProp Part Number in the iam. If you do not want an actual balloon around it, just change the balloon style to use just a leader.
I hope this clarifies your doubts.