Based on its use, it didn't appear to be something very dark. But the name seemed familiar. Then I twigged. CDM is Caramelised Dextro-Maltose. Logic would make DM simply Dextro-Maltose.
A quick search on the internet threw up ad advert from Duttson's, a sugar manufacturer whose name I've often seen in brewing records.
This is the ad.
"MALTO-DEXTRINI'm assuming malto-dextrin and dextro-maltose are the same thing. It's use seems to have been mostly to add body. Or hard to ferment sugars which could be consumed by Brettanomyces.
Is a brilliant Syrup containing 84 per cent.(calculated on the dry matter) of Dextrin and Maltose, which exist as an unfermentable combination; consequently it will be found very useful for increasing the Dextrin in Beers, giving them fulness and adding to their stability.
MALTO-DEXTRIN may also be usedas a Priming, the 12 per cent. of fermentable matter which it contains is gradually split up during Cask fermentation, creating persistent condition.
C.D.M.is FREE from the acrid flavour of Caramel, therefore can be used to advantage in the production of Porter and Stout.
It is not fermentable, consequently can be added to Wort prior to fermentation.
C.D.M.is of hightinctorial power (112 lb. being capable of imparting as much colouring power as a quarter of best Black Malt). When used in suitable proportions as a black malt adjunct produces a soft mellow Stout or Porter, unobtainable when Black Malt alone is employed.
IT IMPARTS iNCREASED PERMANENT PALATE FULLNESS, and a RICH BROWN HEAD.
C.D.M.can be profitably employed; compared with Black Malt it yields a large extract — i.e., 256 lb. (the Excise equivalent for a quarter of Malt) gives an extract of 74 BBEWERS’ lb.
It is sent out in casks of 1 cwt.and 2 cwt."
"Brewing and Malting Practically Considered", by Frank Thatcher, 1898, The Country Brewers' Gazette, page 161.
It makes sense. Most of the beers where Younger used large quantities were ones where you would expect a full body: Scotch Ales and Double Stout, for example. It also explains why some of the FGs are so high.
CDM seems to have been intended to produce a beer with the colour of a Stout, but without the burnt flavour. I've mostly come across it in Porter and Stout. But also in other styles, such as Dark Mild, Brown Ale, or even Bitter.