The riff bears a close resmblance to this 1972 T.Rex B Side, 'Cadillac' It was the B side to 'Telegram Sam' but not so well known. Nonetheless, it's a great song, and maybe Led Zeppelin heard it, and were influenced by it? Bolan was hugely underrated. https://youtu.be/y24B8_YxtQY
One of my least favourite Zeppelin songs - It doesn't have a lot of the passion that I love so much about them: The riff is pretty generic, Plant sings in a mostly hushed tone, and Bonham doesn't seem to have a lot of room to breathe. It resembles the Stone's weaker mid 70's work more than Zeppelin, IMO. Luckily, the rest of Side A, much less the album as a whole, redeems it in every possible way.