Custom SKU not shown when only showing Preferred Models

[worthbak]

STR:

• Add a custom SKU (ex. MK2K3LL/A, iPad)
• Confirm that in-stock results are shown in the inventory list
• Check option to Only show results for preferred models
• Note that Custom SKUs do not appear in the list now

One note is that if notifications appear for Custom SKUs if Notify only for preferred models is checked