I would assume that when the record companies did monthly new releases, the records were available from the start of each month. Given that the book is based on shipments to stores (ie. what the shops were buying, rather than customers), perhaps that makes sense - as soon as new product was available, the stores would order it in.

I believe the CLPGS had access to EMI Archives and other record company data for doing these books, so the information could have been from EMI's own sources. For Decca, I don't know if that's the case, and the CLPGS F series listing has some records which probably weren't even released. It also fails to list Twenty / Summertime In Venice by Gracie Fields (Decca F 10614) - I don't have the book, but looked at it in the British Library. Instead it lists Ballyhoo / Cloudburst by Ted Heath under this number. Take 2 of the latter title came out on F 10624, coupled with Malagueña. I can only guess that the Gracie record was a late addition to the catalogue, and maybe the Ted Heath disc was originally planned for F 10614.

A spreadsheet listing this would be good, which could then be analysed. I guess the Britburn spreadsheet would be a good starting point, and the release dates could then be added in... quite a task, I know! I'd help out if anyone else had a go. Covering the NME chart era would probably be a good start, as that's the era of more interest to us.

So we only need a quick list of these 16 to work out what happend, most likely the sales sheets catching up to reality rather than a planned world