Correct, you have 30 days to send in the second half of DESPP-0419-C, upon completing the firearm.
However, the lower still needs to be transferred through an FFL (or at least a DPS-3 needs to be completed) and part 1 of the DESPP-0419-C needs to be filled out (you need to obtain a serial, regardless of intent to finish).
UNIQUE SERIAL NUMBER APPLICATION (Part 1)
(Pursuant to P.A. 19-6)
? Part 1 of this application is only for the sale/transfer of an unfinished frame/receiver.
?
A completed DPS-3 shall accompany Part 1 of this form and a DPS-67-C if required.?
No one may transfer an unfinished frame/receiver without a serial number obtained from SLFU.? Must be engraved with all markings as required by 18 USC 923(i) prior to transfer.
o To include but not limited to;
? Model (if applicable)
? Caliber or Gauge
? Name of Manufacturer (last name followed by first initial)
? City and State where manufactured
? At a depth of .003 inches
? An incomplete application will result in a serial number not being issued.
? Retain Part 2 of this form until completion of the firearm.
GUN MANUFACTURED COMPLETION NOTIFICATION OR ALREADY OWNED (Part 2)
? Part 2 of this application is to be filled out within 30 days of completion of firearm.
I didn't want to drop the whole text of the Ghost Gun Law (Public Act No. 19-6), but you can find it
here. Start reading Sec5, which is found at the bottom of page 6.