I have also seen it on some Led Zeppelin cover designs, especially on the Mothership album.

Thanks Reynir, at least it's something :)

Interesting topic – I came across the phenomenon a couple of days ago in Vienna, Austria on a building where it said "ERBAUT ANNº 1912" (double underline under the ordinal). Meaning: "built in [the year] 1912", so your Art Nouveau suggestion would fit. The origins might have something to do with ordinal indicators – Wikipedia has quite an extensive article on the a, o and other superscript forms and their function; my guess would be that in decorative lettering this original function was sometimes retained, sometimes wrongly adapted (as in my example, where Latin "anno" would not need any ordinal), and sometimes the form lost the original function and took over another (variation, visual balance, ornamentality)… I'll try to find more examples.

I don't know where it originated but the trend seemed to be swinging from 1880-1910. The Led Zeppelin logo and the calligraphy inside the Zeppelin IV album was based on the same interlock lettering fad. I think they used it to conjure up a mystical Aleister Crowley vibe. Also check the Souixsie and The Banshees logo cover for A Kiss in the Dreamhouse. Have a look at archives of piano sheet music cover art from before and after 1900 and you'll see the evolution of the trend. Piano sheet music cover art seemed like it was a competition to see who come come up with the most outlandish tucks and interlocks. I think the beginning of this trend was the Arts & Crafts movement in situations where a using circular O was thematically appropriate but would have made a title too wide. Instead of squeezing the O narrow and spoiling the theme, the designer would shrink it and add some decorative filler underneath: a bar, diamonds, squares, a teardrop. Articles like THE or AND could be vertically stacked. The horizontals of L,T,Z were used for playful interlocks. The bottom stroke the the K might knock off the stem of a following letter. It was combat! I guess there must have been situations were a designer wanted to pop the O but there was no O in the word so they'd pop another letter like the D in your example. But I don't think it was common. Charles Rennie Mackintosh was a famous designer who used the raised O quite a bit. As well as double or triple crossbars on A, H. It may not have been his invention but it's wouldn't surprised me if he didn't help popularize it as he was so influential in Art Nouveau and Arts & Crafts. The trend seemed to die off with Art Deco and didn't return until those Zeppelin Album covers.