There are 2 ways to approach matrix columns:
1. By index (oColumns.Item(1))
2. By UniqueID (oColumns.Item("1"))
The recommended way is to use the UniqueID.
SAP Business One is committed for a backward compatibility for columns UniqueID.
But don't promise backward compatibility for column indexes. The reason is that we might change the form appearence with versions development.
Therefore, please use only the UniqueID.
You can find the uniqueID for each column by using the "Debug information" option.
The index option is recommended only when you need to run in a loop over all the columns.