For confirmation on Manley's descriptions ...
Item #285 is stated as: "Another example of cracked surface ..."
Item #282 is stated as: "This, like 280, is another example of cracked surface decoration ..."
Item #280 is stated as: "To get this effect on a self-coloured article ..."
So, the item #280 piece is the start for the "cracked surface" decor. The description for that item is quite lengthy but includes ...
All the glass-blower had to do was mark, with some pressure, the outer casing with a sharp piece of metal before the article was blown to full size.
That description shows that Manley did not think it was the type of "crackled decor" as mentioned earlier in this thread. And, presumably, he thought that items #282 and #285 were formed in the same way, although his descriptions leave room for doubt. For example, he did not explicitly state that he thought #285 "cracked surface" was made in the same way, just that it was "another example of cracked surface".
And since I am now in "m" mode (trying to get to the facts beyond the suppositions) I will also add ...
Manley's item #282 actually had as its start to the description:
This, like 280, is another example of cracked surface decoration, but so elaborate that I hesitate to name the manufacturer. ...
In his earlier book for the American audience (
Collectible Glass Book 4, British Glass 1968, second printing 1978) the same item was shown with a description starting: "An elaborate dish, even for Boulton & Mills ...". This clearly shows that Manley was not so rigid in his thoughts and attributions and some people might think. Somewhere between 1978 and 1981, new information caused him to alter his views on that item.
But back to the present ... what was the real "S & W arboresque" technique?