Aging in The Forsyte Saga: how did they do the magic?

Thanks to the good folks who answered me in this thread, I have spent numerous hours in the past several weeks watching the drama of the Forsyte generations unfold.

One of the remarkable things about this 1967 black-and-white BBC production is the aging of the characters. Now, it isn’t hard to see how the men’s hair was grayed and thinned and how makeup scored deeper lines into their faces—Soames (Eric Porter) in particular—nor how a good actor can gradually appear stiffer and more stooped in his gait. They must have used doubles for the wrinkled hands. But how in the world did they transform Margaret Tyzack (Winifred) from a lithe young maiden with clear skin and a slender shape through young matronhood, with a figure that gradually filled out and a complexion that coarsened with time, into a slightly stoutish elderly woman with jowls, wrinkles, and a dowager’s double chin, all looking entirely natural and convincing? I would not have imagined that the technology of the day was up to such special effects, but it is truly hard to believe that this series encompasses only a single television season and not long decades in the lives of the characters.

I tried searching online but was unable to find a behind-the-scenes or how-they-did-it documentary or book on the making of this production that led to the launching of Masterpiece Theatre. Does anyone know of such a thing or have insight into the techniques and artistry of this work?