I remember the first time a strapped on a replica Yorkshire goose wing. I was amazed at how it just fit the point of my right hip, and pivoted so nicely. That pivot instantly suggested a knitting motion that worked with goose wings. Sometimes I still knit like that.
However, it turns out that a goose wing is much more versatile.
If we look at the goose wings in Brears, Fig. 2 No. 13, Fig. 6, & Fig. 7 Nos. 1-3 & 9, the first thing we note is that Brears proveniences them all from a relatively small area of North Yorkshire (Wensleydale, Teesdale, Eden Vale, Balderdale, Appleby, Dent, and Castle Bolton. Despite being from small area the goose wings in Fig 7 are designed to be tucked into a sash, wide belt or waist band, while those in Fig. 2 & Fig. 6 are designed to be tucked into apron strings or a narrow cow band. Thus there was likely a difference in costume of the knitters, suggesting that they were made in different periods.
Brears notes that the earlier sheaths tend to be plainer. My explanation for this is that earlier sheaths were utilitarian (often used for professional knitting). There were like many of them, but they were bought from professional knitting sheath makers, used, and discarded before they acquired sentimental value. Here I have to disagree with Brears because we know from other sources, in the 17th century every village had a cane maker or similar that also made knitting sheaths. The sharp eyed anonymous will point to Fig.5 No 1 and say it is early (1680) and elaborate, but Fig 5 deserves its own post. However, Fig 2 nos 16-18, are early, and have carving that does not detract from their functional use.
Looking at Fig 7 Nos. 4-8 & 10-21 we see knitting sheaths in a spindle style. Some of these are similar to the Dutch knitting sticks that were used with short needles to make knit socks, these are shorter, putting the knitting lower in the lap. Actually some of these would be very functional if they were at least as long as the goose wings. If they are as short as they appear, then they are less functional for serious knitting. From this we can deduce that Brears did not draw at a consistent scale.