Are dubbing brushes a new product for fly tyers?
No! They actually date back to the late 1880s. Shown below is a couple of paragraphs from the book Floating Flies and How to Dress Them by Frederic M. Halford.
For the legs and bodies of some flies the hair or fur of certain animals (dubbing) is used in place of ordinary hackles; and in such cases the plan adopted is to spin or twist it on the tying-silk for the body of the fly, taking care to put an extra thickness on to the shoulder-end of the body, tapering it to the tail, and after the fly is completed to pick out with the dubbing-needle a few fibres to form the legs. There are, however, patterns in which the body is of plain silk or quill, and the legs only of dubbing, and in these instances a somewhat more complicated procedure is necessary.
Thoroughly wax a short length of tying-silk and double it over the dubbing-needle, the point of which is firmly stuck into the edge of the operating-table. Separate the fibres of the hair or fur to be used, and lay them as much as possible horizontally across one end of the tying-silk a, FIG 8; place the other end of the silk over and press it closely down on the fur, FIG 9; then, taking the two ends of the silk between the thumb and forefinger of the right hand, twist up tightly. The effect of this will be to twist up the silk and fur between the two ends of it, as shown in FIG 10, into a rough-looking hackle, which is used exactly like an ordinary one, the silk representing the central quill, and the fur the fibres of the hackle. By the same method, having a hackle of the right colour, but too long in the fibre for the size of the hook, the dresser can, by detaching the separate strands, laying them horizontally across a length of thoroughly waxed doubled tying-silk, and, twisting them up as above, produce an imitation hackle with fibres only half the length of the natural one from which they were taken.
Halford, Frederic M. (Frederic Michael)
Floating Flies and How to Dress Them