From the Evangelical Textual Criticism blog:
[Hoskier] notes that three manuscripts of Revelation have a curious little drawing (or perhaps better a pictogram) instead of the word ‘the sun’ ‘ο ηλιος’.
…It is a curious little drawing and I am not sure it looks like a sun, much more like a circle with a tail, perhaps a comet.
Does anyone know what this about, are there other examples, other symbols?
And a number of folks chime in in the comments.