Hi,
Ok, now I understand that the "exact" mode on one field can't work for what you want.
If you select another mode, like "any word", then if you search for 123, you'll get your result as expected.
But rereading this thread from the beginning, I understand that the other modes won't work since you want the results to appear only when the full serial number is entered.
An alternative I can see is to create 10 or 20 custom product fields (serial 1, serial 2, serial 3, etc). Basically, for as many serials a product can have as possible.
Then, select all these custom fields in your filter, and in each product, enter only one serial for each custom field. That way, you can keep using the "exact" mode and it will meet both of your requirements.