Neckless and proofless?

[The Squatter of Amon Rûdh]

While reading The Hobbit this weekend, I was struck by an occurrence of 'neckless' for 'necklace'.

'I beg of you,' said Bilbo stammering and standing on one foot, 'to accept this gift!' and he brought out a neckless of silver and pearls that Dain had given him at their parting

If Tolkien had put it there, clearly there would have to be an explanation for such a use, so naturally I cross-referenced the copy I was reading with my other copies. I found that two of them have this error, but that three others do not.

What makes this interesting is that the copy in which I first found the mistake was a fourth impression of the fourth edition (George Allen and Unwin, 1983) and the only copy that duplicated it was a 1978 Guild Publishing edition that is identical to the GA&U edition even down to the pagination. However, I have an old Unwin Paperbacks copy of The Hobbit from 1983 (a late impression of a 1975 paperback edition) that does not contain the mistake. How, I wonder, did a mistake like that get into such a late edition?

I would be very interested to know if this was a mistake in the typesetting for the Allen and Unwin fourth edition, and for how many impressions and in which editions it continued. It seems clear that the mistake was not present in any copies before 1978, but I hope that the rest of you can confirm this from your own libraries.