Custom Field not available inside Inventory ID selector

Hello, the following code snippet does not result in my new field being added to my inventory selector. This seems like a straightforward approach?

namespace PX.Objects.IN
{
    public class InventoryItemExt : PXCacheExtension<PX.Objects.IN.InventoryItem>
    {
        #region UsrModelID
        [PXDBString(125)]
        [PXUIField(DisplayName="Model Number")]

        public virtual string UsrModelID { get; set; }
        public abstract class usrModelID : PX.Data.BQL.BqlString.Field<usrModelID> { }
        #endregion
    }

    [PXNonInstantiatedExtension]
    public class IN_InventoryItem_ExistingColumn : PXCacheExtension<PX.Objects.IN.InventoryItem>
    {
        #region InventoryCD
        [PXMergeAttributes(Method = MergeMethod.Append)]

        [PXCustomizeSelectorColumns(
        typeof(PX.Objects.IN.InventoryItem.inventoryCD),
        typeof(PX.Objects.IN.InventoryItemExt.usrModelID),
        typeof(PX.Objects.IN.InventoryItem.descr))]
        public string InventoryCD { get; set; }
        #endregion
    }
}