Honestly, I don't think I have any such documentation anywhere in my files for any of the guns I own.
Seriously, you may want to consider getting such doc. together. May take a while -- requesting receipts and whatnot -- but surely worth it. Also suggest you scan and upload the docs once you have them.
Technology is going to make it much easier for the law to track down "undocumented" guns and confiscate them. Use technology now, for your advantage.
I live in NYC, where you're not allowed (I've heard) to keep handguns in your apartments. The NYPD will rent you free locker space for your handgun (gee, thanks).
I don't own a gun, but have contemplated it. First thing I would do is get first-class training on handling, shooting, etc. Second thing I would do is buy the gun. Third thing I would do is immediately get my paperwork in apple-pie order.
I have no paper on any of my arms. I can't imagine what "technology" it would be that could track any of them to me. But I will concede the point and entertain the hypothetical scenario that the "authorities" have come to confiscate them. My sympathies to the paramedics and the cleaning crew that follows them.