If [http://ziggr.com/heisig/](http://ziggr.com/heisig/) is an authoritative list for the 3rd edition, and we consider only pre-2010 joyo kanji, then it looks like it’s 璽 (I wrote a little computer program to check.)
Looking around the internet, I hear ≥6th edition RTK *does* cover all the post-2010 joyo kanji.
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If [http://ziggr.com/heisig/](http://ziggr.com/heisig/) is an authoritative list for the 3rd edition, and we consider only pre-2010 joyo kanji, then it looks like it’s 璽 (I wrote a little computer program to check.)
Looking around the internet, I hear ≥6th edition RTK *does* cover all the post-2010 joyo kanji.