Hi Dave,
can’t help you with the reasoning behind the choice for reprinting but I’ve also got the vol. 68 and also volumes 1 to 12. Volume 1 was reprinted in 1976. I think you’ve been able to buy bound copies of complete volumes from quite early on ( I seem to recall adverts for them) As well as back issues, you could also buy the complete volume less adverts. The earliest bound volume I have is volume 48 and that looks as though it was printed as a complete volume rather than individual issues just bound together.
The earliest issue in my collection is volume 13 issue 235 ( October 26th 1905) and that has plenty of adverts! Incidentally, the price was 2d or 3d to include free postage.
I would imagine that the publication has always contained adverts?
John