This motive is also related to a passage referred to by multiple Wagner scholars as the “Tantris Motive,” which actually sounds a major third higher in the music (Pfohl, p. 217).  Scruton doesn’t refer to the above-mentioned motive, but claims (along with Redlich) that Wounded Tristan is a variation of Longing because the final three notes of the two motives move down by half step.

Motive Labels
“Tristan Wounded” – (Lavignac, p. 292)
“Sick Tristan”
“Isolde’s Narrative”– (Kobbé, p. 117)
“Motiv des siechen Tristan” or “Motive of the Ailing Tristan” – (Windsperger, p. 40)
“Tantris” – (Hight, p. 278)

Wounded Tristan – Order of occurrence by Act and Scene
Act I: Scenes 3-5
Act III: Scenes 1-2