I think that is what is going on, too. I have never seen a shed finished this way, but it might make it easier to convert.
Board outlined in yellow: it is flush with the bottom of the fascia board, right? Two nails boxed in red help hold it?
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Same board boxed in yellow, nailed to the underside of a rafter with nail boxed in red? Yellow marked board looks flush with bottom of fascia board in this pic.
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You'll have to take a good look at how that board is attached, but if you can get it out, looks like you are set for soffit vents.