I am using MB21, which has a number of items on the same page. Is it possible to know which item number I am choosing when clicking (or double-clicking) on the field number?

Yes it is. Please use the system fields:




 - You can use the Debug window to see the values of these variables when the InputScript starts. 
 - The following is a sample InputScript to show the field value of the clicked entry field:

Set V[value] "&F[&V[_cursorfieldname]]"
Message "you clicked in (&V[_cursorrow],&V[_cursorcol])\n\n Field value=&V[value]"