Maggs Bros. used markings which are paricularly helpful to those with an interest in provenance, because they usually indicate the date, and often the source, of their purchase. Usually there is in the top right-hand corner of the lower pastedown a letter to indicate the source ("S" for Sotheby's, "C" for Christie's, etc.), followed by a month and year, such as "6/89" for June 1989. Towards the lower right-hand corner there will usually be a price-code, formed of characteristic joined-up lower-case letters, often ending with one or more "S"s, representing zeros.
In this case, Maggs appear to have acquired the manuscript for a second time, and thus supplied a new purchase price to replace the older one:
In this example, the date is (unusually) placed with the price-code and (also unusually) in upper-case letters. The date is sufficient to suggest that the manuscript had been bought at Sotheby's in a June 1989 sale, and indeed a quick search showed that it had been lot 48, bought by Maggs for £2,000.
Here the date is December 1973 and the price apparently £2500; the price-code is in the far more common rounded lower-case lettering: