Propagation of ListFillRange property

[KMC1]

PROBLEM:
I'm trying to utilize VBA (Excel 10) to propagate "ListFillRange" properties

SETUP:
The are two columns, labeled: "Select JOB#" & "Select DRAWING#"
Each column contains (23) ActiveX Combo Boxes
Each "Select JOB#" Combo Box [perfectly] lists current jobs
Each "Select DRAWING#" Combo Box perfectly lists (+/- 200) pertinent drawings ONLY when I MANUALLY fill in the "ListFillRange" property

However, I would like to fill in the "ListFillRange" property based on the "Select JOB#" selection.

Below is typical VBA coding that I have tried:

Private Sub ListedDwg1_Change()
ActiveSheet.Shapes("ListedDwg1").Select
With Selection
    .ListFillRange = MDL_1       'Current try
   '.ListFillRange = "$R$16:$R$16"     'Previous try
   '.ListFillRange = "$AV$1000:$AY$1700"     'Previous try
End With

Even by entering a named range, "MDL_1" (Current try), the "Select DRAWING#" combo box displays only "MDL_1" (or "$AV$1000:$AY$1700",or "$R$16:$R$16") instead of a list of (200) drawings.

How can I get the respective list of drawings to display in the respective Combo box?

(I just clicked the ListedDwg1 Combo box & all it shows is "MDL_1" with no additional drawing numbers)

Thank you for your assistance !!!