Just to be frank, I think Sad Wings is pretty inventive and you'd be hard pressed to find a riff such as The Ripper in a 'mainstream' release before it.
I do not know about punk rock movement much, but having a single staccato powerchord seems like a usual punk trope for verse music. Priest might have picked it up there (the period aligns), or just from some common ancestor. But they've pinned it down to the beat, every beat and were using it for its texture and vibe. Like a synth pad that provides atmosphere, repetitive and even rhythmic quality. That kind of a thing became a staple of metal music soon after.
But, the sound is not quite as energetic as less palm muted, more open, Rock Bottom or half syncopated Light in a Black. These examples, for me, sound better on the gear of late 70s because we're not talking about high-gain amps. Palm mute digging into a riff, the chug, it's just not gonna have volume and drive if the amps aren't high-gain.
For this amp theory, check out the difference between Prowler 80 and 88's main riff. They're not the same. The original riff is not palm muted.
TLDR; Priest Sad Wings riffs are very modern metal, but the amps of the day don't support it in enough volume. The drums are aligned to this fact, light hits, very groovy sound. Halford's voice too. So the record does have metal riffs, but it's less energetic in execution then some "blues based HM" like Rising.