Just a couple suggestions:

>RowSource = Devices.Serialno,dModelno,DeviceCode
>RowSourceType = 6 - Fields
>ColumnCount = 2 && should be 3?
>ColumnWidth = 100,100,0
>BoundColumn = 3
>DisplayValue = Devices.Serialno

>and the following code in these methods:
>INIT:
>THIS.VALUE = (Devices.DeviceCode)

Problem with This.Value = Devices.DeviceCode could be that DeviceCode is numeric?

>Then, in the INTERACTIVECHANGE:
>THISFORM.REFRESH()

InteractiveChange of a ComboBox fires every time the highlight moves. In other words, if you drop down the combo, each time you hit an arrow key, InteractiveChange will fire. This will fire the Form.Refresh code way more than you probably intended. You probably wanted THISFORM.Refresh in the Valid or LostFocus event, so the form refreshes after the user has confirmed a different selection. But I could be wrong.

I know this doesn't answer all of your questions, I hope it answers some.

Cheers,
Rich. (Also still more of a 2.6 programmer...)