Thanks Torrey.
I don't think it is a floor sweeping, error or prototype as the person I got it from used to play poker and had a collection of odds 'n' sods although they didn't know how or why they had it as they had never played there (they did have a Vic SC one too though).
As I mentioned, I know of another 50p without inserts but it is the red Playboy (original London, CG073063) one that should have 4 black ones (as you know) so I think it may have been a cheaper alternative without inserts perhaps but just a guess really. I did mention it to the authors of the "book" a few years ago along with a few other "corrections/additions" but had no reply.
Obviously, it could well be and hopefully somebody knows something about it (with "facts" to help clear up the mystery and not just speculation) that will help fill in the gaps.
That's the main reason I put it up, to see if anybody else has one or knows anything that could be of use plus to let people know that new finds do still exist with the UK Playboy ones.