Chevron cassette shells - what's the story?

This week, I spotted a copy of the rare Chevron cassette, Top Hits of the Year (1978) on EBay. Although I bid on it, I was outdone by someone - and I wish now that I'd bid higher, because this example differs from the one I already have. This is the copy which was sold on Ebay:

What's unusual about it is the cassette itself, which is inside a black shell, with a white paper label stuck to it. Mine has a white shell, with the details printed directly onto the plastic:

The insert appears the same however; this is my one:

Discovering this alternative "pressing" (if that's the right word to use for cassettes!) prompted me to go through the rest of my Chevron collection and see if there's any pattern to this. The tape above is, of course, assembled from the "Parade of Pops" series, and constitutes one of the end-of-year compilation albums. I knew I'd definitely seen black shell editions of "Parade of Pops" before, but couldn't remember which.

Although my collection isn't quite complete, this is what I found: Cassettes up to and including CHVP 21 (1981) have light grey shells with white labels, but from CHVP 22 (1981), the white shell with direct print becomes standard. The end-of-year collections, Top Hits of the Year, not surprisingly tend to follow the current format.

However there are a few exceptions: CHVP 2 (1978) has a white shell with the printing directly onto the plastic, whereas CHVP 3 (1978) has a black shell with a white paper label, of the type featured above. And there's another one too - CHVP 15 (1978), which is another of these black shell editions.

We might understand the inconsistency early in the series - Chevron presumably had a short-term contract with the manufacturer before the series was fully established, and might have been trying out different companies. But why does CHVP 15 have a black shell? And why does the one above come in two different versions, neither of which is in the then current style?

Please let us know if you can shed any light on this - or if you have any more unusual editions from the series.