Sorry to resurrect this old thread, but I was looking at ‘Early Buddhist Theory of Knowledge’ and it looks like the book sometimes translates the Pali words in question differently in different places in the book. Though, to be honest, it is difficult to be sure since the Pali isn’t shown in all cases.
Page 60: seen, heard, thought of, understood or attained
Page 92: seen, heard, sensed or known
Page 345 and 381: seen, heard, sensed, thought
None of these exactly matches the sequence you translated them to in Ud 1.10: seen, heard, thought, known. The sequence on page 60 appears to comes closest.
If merely the seen and merely the heard means seen and heard without the embellishments of apperception/sanna than I don’t think thought makes sense. If sanna has ceased, there is no thought so merely the thought makes no sense. Merely the sensed would make sense on the other hand.
If merely the seen and merely the heard means seen and heard without attachments thought could make sense, but then merely the known does not seem to make sense. Thought comes after perception so the known may come without attachments, but being accompanied by thoughts hardly seems like merely the known.
We do know that in Ud 1.10 there is no ‘you’ in that. If merely the seen etc… means without a seer etc… merely the thought without a thinker seems to make sense except that merely the known still seems off for the reasons stated above.
Given the above, Ud 1.10 seems to be describing the state where sanna/apperception has ceased so seen, heard, sensed, and known seems to make the most sense, to me anyway.