Horse 1225 - Charlie And The Error Factory

I was flipping through a copy of that perennial favourite children's book "Charlie And The Chocolate Factory" when I've either noted something of a continuity error, or perhaps something deliberate.

In the edition I'm looking at now on page 51, Grandpa Joe gives Charlie a sixpence to go out and buy a Wonka bar so that he might win a Golden Ticket. Just 7 pages later on page 58, Charlie finds a fifty-pence piece on the ground which he then uses to buy the Wonka bar which ultimately does yield a Golden Ticket.

Spot the error?
Why on page 51 do we have a pre-decimal 6d, but on page 58 a decimal 50p?

The front of the book tells us two dates for publication. The book was originally published in 1964, and the second edition was first published in 1973.

If in the 1973 edition we are told that Grandpa Joe hadn't gotten out of bed in years, then it's entirely feasible that he might have a pre-decimal sixpence, considering that Britain went Decimal in 1971. Decimal coins though started finding their way into circulation in 1968 and pre-decimal coins were supposed to be demonitised in 1971.
What I'm wondering is was this a deliberate inculsion in the 1973 edition or was it an oversight?


It also leads me to ask what Charlie found on the ground in the 1964 edition. Presumably he would have found a half-crown (2/6) but having never seen a 1964 edition I don't know. Certainly Roald Dahl would have had no way of knowing that in seven years time, the half-crown would not be in use.

I also wonder what sort of game Roald Dahl was playing at in 1964. After looking through the archives at the Mitchell Library this afternoon, I can tell you after looking in a copy of The Times, that in 1964 a Mars Bar should have cost threepence; if Charlie paid sixpence, he was being ripped off.
I also looked through a 1973 copy of The Times and found that a Mars Bar was worth 8d. Now the old Shilling became 5p, which also means that the old Sixpence was only worth 2½p and wouldn't even come close to buying a chocolate bar.

In either time period, the amount of money which Charlie spends isn't true to life; on top of this the book isn't even consistent with itself.

Still, I suppose I shouldn't expect much from a piece of fiction which is 47 years old and in which in the 1964 edition had instead of Oompa-Loompas had "Black Pygmies" working in the factory in what amounts to slave labour.