I believe record companies were in the habit of releasing new records in batches, so that might explain why new entries occur mainly together. It's a shame that they don't make it abundantly clear where the information has come from, but perhaps they felt doing so would have opened them up to a unwanted level of scrutiny (or maybe they're not quite sure themselves!). But as you say, it's the best we have, so we'll have to live with it.

I must say I haven't come across any instances of "no particular artist named", but then again I'm using the second-hand information on this website: hitsofalldecades. I was assuming it was a faithful transcription of the book, but perhaps it's slightly different.

By the way, what are the 6 records that no-one has ever seen?