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@joshuabarksatlcs wrote:

See Rev Proc 2017-34. 

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I think you meant Rev Proc 2013-30, right?

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@joshuabarksatlcs wrote:

2.7.  @TaxGuyBill  said: But I think 2020 has already been filed as a "C", it would mess up the ability to file the late S-election.  Source?

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Rev Proc 2013-30.   🙂   And the shareholder's signatures on Form 2553.

Section 5 covers it.  Here is an excerpt:

(5)(i) The Requesting Entity timely filed all required federal tax returns and information returns consistent with its requested classification as an S corporation for all of the years the entity intended to be an S corporation and no inconsistent tax or information returns have been filed by or with respect to the entity during any of the taxable years,