ATF regulations allow completed form 4473s to be filed by a TTSN (Transferor's Transaction Serial Number). When filing completed form 4473s by TTSN, the TTSN must be entered in the area at the top right of the first page of the form 4473 and the TTSN must also be shown in the disposition record for the firearms listed on your form 4473 in your bound book report. Nothing else is allowed to be entered in this field. If you are not filing by TTSN, then this field should be left blank. A TTSN should never be an Item Number or Serial Number of any firearm.
A completed form 4473 is a form 4473 that has been filled out by the buyer and the seller, and the firearms have been transferred to the buyer. If the buyer was denied or the form 4473 was not completed for any reason then you do not put a TTSN on that form 4473.
You should NEVER issue a TTSN for any transaction other than when a form 4473 was completed. If FastBound is showing you a warning message or not allowing you to enter a TTSN then that disposition does not allow for one. If you have a TTSN entered and FastBound is warning you that it needs to be removed then you should either remove it or move the entry to a different field before you can save.
You should NEVER issue or write in a TTSN on the form 4473 until after the form 4473 has been completed and the firearms are being logged out of FastBound.