How have you got the Serial number field setup? Do you have Auto-enter Serial set on that field?
If so make sure the 'Next Value' option has the required number of zeroes before it, i.e. 051.
Another option is with the calculation you are using to concatenate the fields to create you 13 digit number you could reference your Serial Number as follwos
Right ( "000" & SerialNumber ; 3 )
This will place 3 zeros before your serial number and the extract three characters from the right, leaving you with 003 for example.
I hope this helps
doh - so simple!!
thanks Orlando - i just needed to add the zeroes in the manner you described.
I already used the method you described for the concatenation...it was just the Auto Enter wasn't set quite right.