I'm not a lawyer. This is my current understanding as I read the text of the bill.
Basically, starting January 1, 2018 you will be required to contact the DOJ BEFORE you begin a build to have a background check and be given a serial number.
As of January 1, 2019 ALL firearms other than a number of exceptions like C&R longguns, unloaded antiques, DDs, etc will require one of two types of identifying information - either a state issued identifying mark or markings pursuant to Title 26 of the federal regulations.
So it's my understanding that IF you make your receiver prior to January 1, 2018 AND mark your homemade gun according to Title 26, then it will not require registration.
So if you only put a serial number, or mark it with an electro pencil, or leave information like your name off of it, or do anything else that fails to meet the full criteria of ATF marking as per a manufacturer, then uou will need to get the serial from the state DOJ. Again, I'm not a lawyer, that's just what I understood the bill to say.
Of interest to note is that C&R longguns are exempt but not C&R handguns so it sounds like a C&R handgun without serial will need to be engraved unless it has sufficient markings to track it. Also, the bill notes making and assembling separately, which means there could be a requirement for someone who buys a stripped receiver from an FFL to get a serial number as a builder and put that on as well.