I still don't get it.

When the wait time was up to 7-8 days and I had some lowers due in, I'd gone in and filled out 4473 while guns were still in transit, not yet received by the FFL.

I'm pretty sure the FFL submitted the BGC without any item serials, possibly even without descriptions. The BGC is also good for 30 days, so guns can be added to it at any time.