If so called UBC passed literally every gun manufactured after that date would effectively be registered. Either the trail could be followed to the current owner/possessor, or if not, the end of the trail would be in violation of the law for having transferred possession without a background check and paperwork.
Also any gun that was lawfully transferred after that date would then be effectively registered.
Virtually every new gun sold since 1968, when the use of a form 4473 became mandatory, would be what you claim is "effectively registered", since new guns are most always sold thru a licensed dealer. At the very least, every new gun sold since June of 1996 is still on the official books somewhere. Then there's the secret registration books those guys with black suits and wearing sunglasses keep of records supposedly destroyed. Thus, if one is so fearful of the gun registration/confiscation theory, it's already too late....unless you haven't bought any new gun for almost half a century. Without the paper trail to show you no longer have the gun, odds are you will be tortured or given a truth serum(or both) until you give up said guns or die. Then there's the way they track credit card purchases for ammo, reloading components and anything else gun related. Even without purchasing a gun or ammo, those security cameras affixed to every streetlight and mailbox will see you walk in the house with a package that could be a gun. Odds are the camera has X-ray vision to see what's in the package, thus, they got you, coming and going. As long as one is fantasizing, you may as well go all out, cause that's what thinking UBCs are only intended for under the table registration is....a fantasy. If you cannot trust those officials that you and your peers have elected, to keep
all your rights safe, you have more things to worry about than just your guns and the RKBA. If the majority of your peers vote for someone that endorses gun confiscation, you are screwed even without UBCs. If that day comes, the Storm Troopers will march you out of your house and burn it along with the guns, cause those same friends turned you in.
Now.....getting back to reality.
While I too doubt very much that making UBCs the law of the land will reduce crime and Mass murders to any great extent, I also don't see the possibility of confiscation being anything close to realistic. What I do see as realistic, is the change in attitude since the last mass murder, to attempt in any way, to keep certain folks from obtaining firearms. This attitude ebbs and flows, back and forth, depending on the frequency and horribleness of violence when guns are used. In several of the latest polls from various institutions, 9 outta 10 Americans want expanded background checks.....
When asked about specific policy steps, 92 percent of respondents favored expanded background checks, 87 percent supported prohibiting convicted felons or people with mental health problems from buying guns and 85 percent were in support of banning people on the terror watch list or on the no-fly list from buying guns. Only 9 percent of those surveyed supported preventing all Americans from owning guns.
This tells me the majority of Americans do not want to take guns from law abiding, responsible gun owners. They just want to prevent BGs from easily and legally obtaining them. Again, will UBCs help? I dunno.