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"P-Return of contribution taxable in 2017"  UNTIL you get to the next screen and it asks if it is a 2018 or 2019 1099-R- say 2019 and then it means "P-Return of contribution taxable in 2018" - the year advanced by 1.

BTW: this ONLY means that you are adding the excess to the 1040 line 1 wages so that you will not need to amend next year to add it to the 2018 tax return a year from now when  you receive the 1099-R.   This has NOTHING to do with not being able to e-file with a W-2 excess - reporting the excess does not change that.  You still must print and mail, and the W-2 must still report the excess.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**