you keep missing the most important part of that statement which is WHEN YOU DISPOSE OF IT. Not when it becomes C&R, only when you dispose of it do you put it in the book if its not already there.
You don't have to look at any book to know which of your guns are C&R, if they are at least 50 years old, they are C&R. If they were made before 1899, they are antique and don't need to be in the book. There's only a rare few examples that are younger than 50 but still C&R eligible which you need to check the book for. The book is NOT comprehensive.
Right, the book is not comprehensive, there are guns that are C&R that are not in the book.
But there are *NO* guns that *are* in the book that are *not* C&R. Every single gun listed in the book is a C&R, even if it's not a complete list.
And, you're missing my point:
Say you have a gun. It was made by a small production one-man manufacturer. It's 20 years old. It was made by $someone_famous.
Then someone else with a similar gun thinks it's *SO SPECIAL* they petition the ATF to make anything made by $someone_famous a C&R, because the gun's value is more to do with who made it than what it does. (One of the possible criteria for a non-relic curio)
Now you want to sell that gun, which has become a C&R while you owned it, even though it's less than 50 years old.
If you don't look in the book periodically to make sure that anything you sell isn't there, you would accidentally sell a C&R without putting it in your book, because you didn't check the ATF book!
The argument, "that almost never happens" is a bullshit way to interpret law. Either it's a requirement, or it isn't. "a few rare examples" doesn't protect you from prosecution.
The law says you must "enter into the A/D all curios and relics acquired
after receipt of the Collector's license." (bold added for emphasis) That's a direct quote from the paperwork I got when I got my C&R. Anything that becomes a C&R was *NOT* "acquired after receipt".
As far as the "when you dispose of it" part, why do you think this applies to guns that you were never required to put into your bound book to begin with?
And how is it even possible to comply with the "...be recorded no later than the close of the next business day following the date of such purchase or other acquisition" clause for a gun that wasn't a C&R when you got it, or if you didn't have a C&R license at the time?
The fact that it's literally impossible to comply with 27 CFR 478.125 for any gun that wasn't a C&R when you got it makes the the notion that you have to enter it into your book immediately before selling it ludicrous.
That's why I'm looking for a specific citation on guns that you had before, or became, C&R guns.