I strongly recommend that you NOT use such a lot number as a primary key to link tables in relationships.
You can set up cMonth as a calculation that computes a date for the first day of the month from a date field in your record:
DateField - Day ( DateField ) + 1 )
You can then set up this self join to match a record to all other records in the same table from the same month:
YourTable::month = YourTable|SameMonth::Month
YourTable|SameMonth would be a new table occurrence of YourTable.
You can then define a number field for the sequence part of your lot number and then this calculation can be used to assign the next value in the sequence:
Max ( YourTable|SameMonth::Sequence ) + 1
Then you can set up a field with an auto-enter calculation:
Right ( Year ( Datefield ) ; 2 ) & Right ( "0" & Month ( Datefield ) ; 2 ) & Right ( "00" & Sequence ; 2 )
And you can set up a "unique values" validation option on this field to ensure that you don't accidentally get two lots with the same value.