I have The works of Charles Dickens (for forty years) in Thirty volumes, missing 6 and none have publishing dates.
I think the simple answer is that many publishing houses, in those days, simply didn't include the publishing date. Reprints would include the reprint date though. We know around when it was written but an exact publishing date was not always made public.
Dickens both added to his serials and edited them throughout his career. Many of his works were done in sketches, much of his work was initially published as serials in monthly installments. This is very possibly the reason for missing publication dates.
