Two items actually -  
 
When you look up a serial number in SC-A, (enter down to the serial number line, do a lookup by serial number) the lookup screen shows the sales order number - but after you select it, the SC-A screen shows the Sales order number as 0 and the sales order number is in the Invoice number box.
 
Second - it would be nice to have the invoice number in the grid for SERIAL - But the information is not in the Serial.B file.  Any way to add it to the grid and then have it populate the main SC-A screen in the invoice number field?