In this section of HB4383 (https://www.ilga.gov/legislation/publicacts/fulltext.asp?Name=102-0889) it says "undetectable firearms", aka polymer type, have to have serial on a steel plaque (P80 frame in pic below, lower right). Pistol P80s & the like have one embedded in the dust cover but it's not very big. SIG P320 fcu (yes there are 80% SIGs too) available area is even smaller although the whole fcu is steel. If one has already serialized their 80 frame/receiver there's no room for more. If it's a metal gun like 1911 or AR series there's room to put wherever desired but what happens when a firearm now has 2 serial #s? If an individual obliterated an existing self applied serial to then apply one conforming to state/fed "ghost gun" requirements does that not open a huge can of worms?
(f) Unserialized unfinished frames or receivers and
unserialized firearms serialized pursuant to this Section
shall be serialized in compliance with all of the following:
(1) An unserialized unfinished frame or receiver and
unserialized firearm shall be serialized by a federally
licensed firearms dealer or other federal licensee
authorized to provide marking services with the licensee's
abbreviated federal firearms license number as a prefix
(which is the first 3 and last 5 digits) followed by a
hyphen, and then followed by a number as a suffix, such as
12345678-(number). The serial number or numbers must be
placed in a manner that accords with the requirements
under federal law for affixing serial numbers to firearms,
including the requirements that the serial number or
numbers be at the minimum size and depth, and not
susceptible to being readily obliterated, altered, or
removed, and the licensee must retain records that accord
with the requirements under federal law in the case of the
sale of a firearm. The imprinting of any serial number
upon a undetectable firearm must be done on a steel plaque
in compliance with 18 U.S.C. 922(p).
I skimmed through 18 U.S.C. 922(p) (https://www.law.cornell.edu/uscode/text/18/922) and could find no reference to marking on a steel plaque whatsoever, though it's possible I missed it. There is a section talking about a "Security Exemplar" made of 17-4 stainless that is used for calibrating detectors, and 2 references to the word "serial" ("has had the importer’s or manufacturer’s serial number removed, obliterated, or altered..."). 27 CFR 478.92 addresses marking requirements re:size, depth, etc. but also doesn't address the metal plate thing for "undetectable" firearms. Is the metal plate thing an ATF ruling?
I wonder that if a serial is already there that the HB4383 marking doesn't come into play. Searching the text of the law the words "unfinished frame" is used 18 times while the word "finished" is only used once in this section with regards to logging in the firearm. I can see how it could be interpreted that as long as the gun is marked per ATF you are good to go, but I'm not a lawyer and this is just my interpretation.
(3) Every federally licensed firearms dealer or other
federal licensee that engraves, casts, stamps, or
otherwise conspicuously and permanently places a unique
serial number pursuant to this Section shall record it at
the time of every transaction involving the transfer of a
firearm, rifle, shotgun, finished frame or receiver, or
unfinished frame or receiver that has been so marked in
compliance with the federal guidelines set forth in 27 CFR
These are my mental rambling and interpretations so don't take anything as a guide for what you might do.