When Collins assumed the publishing of the Agatha Christie books in 1926 the standard printing sequence was to sell the new first edition for 7/6. First editions often had multiple print runs, so still technically still first editions, but later printings. Once demand died down, second editions were printed where the size or cover art changed, and usually had a drop in price to 2/6 but were still full size hardbacks. Of note, Bodley Head's first price cut was to 3/6, before going to 2/6 so perhaps Collins was trying to be more competitive in the market. After the 2/6 editions, Collins went to a smaller size book (6 inches by 4 inches) - at a cheap 1/- price - essentially the precursor to a paperback (not yet invented).
The first two books to be sold in the 1/- format were Collins first two Christie's - The Murder of Roger Ackroyd and The Big Four. They differ slightly from the later 1/- editions in that they did not have advertising on the rear panel. The Murder of Roger Ackroyd is of particular appeal because the cover art matches the first edition. The Big Four is stylistically the same art but with a cream, not blue, background. While undated, the general assumption is that these were printed in 1928.
Collins then started placing advertising from Kolynos Dental Cream on the rear panel of the jacket and on the rear board of the hardback. Collins printed several books in the 6x4 inch size with the advertising, including Partners In Crime (which also replicated the original cover art - one of my favourites), Murder of Roger Ackroyd (again) and The Mystery of The Blue Train. These small format Kolynos versions are assumed to be mostly from 1929-1930.
The next iteration of these cheap 1/- books can be seen in the 1933 printing of The Murder At the Vicarage. It is a little larger, measuring 7 inches by 4 1/4 inches) but still priced at 1/- and continuing to feature the Kolynos Dental Cream ads on the wrapper and rear boards.
The End of An Era: Paperbacks arrived in 1935 at a price of sixpence - so half the cost of a cheap 1/- hardback. As a result this short period from 1928 - 1934 appears to be the only time Collins printed these books.
Collectiblity: These are very collectible and the early reprints are a great way to own Christie's printed by Collins in the late 1920s and early 1930s. They provide the collector access to original cover art which is otherwise generally unavailable - whether literally or financially at a much more reasonable price point. Even though they are reprints, the dust jackets are still very uncommon. Collectors who enjoy these period items should consider acquiring what is available and affordable. Recent sales of the earliest version have generally been in the $500-$600 range for very good condition books and wrappers. The later versions with the toothpaste ads are slightly more common, but still usually command prices in the $300 range.
Footnote: For those not familiar with pre-decimal coinage in the UK, a price of seven shillings and sixpence would have each shilling converting to 5p in modern coinage and there were 12 old pence in one shilling, so a sixpence was 2.5p... thus 7/6 was 37.5p decimal!