The images which follow show the rich variety of trade terms for top-fermentation beers, especially ales, in America before WW I.
Broadly speaking, there was cream ale, stock ale, half-stock ale, musty ale, old musty ale, India Pale Ale, XXX Ale, Burton ale, and yet further variations on these themes. Some very old ages were advertised for some stock ales and Burton ales, see eg. here and here. At least one newspaper account referenced a “Bass’s old musty ale”, however the musty moniker was probably an American addition, it seems doubtful Bass of Burton used the term.
The non-retail trade used the term “fresh ale” to mean an ale intended for quick sale and which would sour within a week or so failing sale. See this New York State court decision involving a brewer’s attempt to gain damages from a creditor of a customer who stored stock ale for the brewer due to lack of space at the brewery. The only terms used in this decision are fresh ale and stock ale.
The brewer’s terms “present use”, “lively ale”, “still ale” were largely intra-mural although isolated examples can be found in trade ads. (Of these present use was most frequent but this was from 1860s-1890).
Musty ale may have been made to a specific technique such as mingling stock ale and new production, or stock ale and fresh lager (the kind used for krausen), or stock ale and partially-fermented wort. Maybe some musty ale was made by adding a tart stock ale to an ale fermentation or in some other way during mashing or brewing but I incline against this.
I don’t think musty ale was cream ale as such or lively ale. Some ads show both forms, for example. In the ad linked, Smith Cream Ale, probably Robert Smith’s of Philadelphia, is shown above a “musty ale”. While it’s possible they were from two different makers, it is unlikely especially as Robert Smith produced a musty ale. See the ad in this posting from last year by Jay Brookston where a musty ale is shown, also a Burton ale, and numerous others. While cream ale is not shown, the XXX shown was likely its cream ale. Finally, this news ad from 1910 seems to clinch the matter.
Musty ale seems to have had a fresh character as part of its make-up. This is attested by this 1920 brewing record in Virginia, a home-brewing contrary to the Volstead law. The brewer called his ale, ready in nine days, a “good musty ale”. It’s a normal brewing, using malt extract and sugar in this case. Why would the brewer have likened it to musty ale? Presumably due to its fresh, yeasty character. That could have been achieved in the commercial brews in a number of ways, as indicated above.
However, half-stock ale perhaps was the same as musty ale, or some musty ale, since it was obviously a mix of fresh ale and aged (stock) ale.
The musty ale in the ad below may have been a blend of the brewery’s crystal ale and stock ale.
Note re images: The first image above was sourced from HathiTrust. The second was sourced here. The third, here. The intellectual property in or to these images belongs solely to their lawful owner or authorized user. Believed available for educational and historical purposes. All feedback welcomed.