Not sure what the 20 next to Code D in Box 12 of your W-2 would be. Code D indicates contributions to a Retirement Plan, so be sure to enter the amount in TurboTax. Just choose 'D' from the dropdown menu.
It could be an employer designation for the type of Retirement Plan, if your employer offers more than one type of plan. You could verify this with your employer.
Click this link for more info on How to Manually Enter a W-2.
All that means is that the contribution was for 2020.
In the past, occasional employers have issued W-2 forms that for some reason also show the prior year's contribution (a full year before)....thus, if on your 2020 W-2, you also saw a D 19 $4500. all that meant was that $4500 was contributed to the 401k in 2019.
But a D 20, for a 2020 W-2 for the normal contribution being recorded on-time.....the "20" would seem to be unnecessary.
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I have no idea if they also mess with box 1 when they show the extra prior year's entry...nor why they do it that way at all, unless, for some reason the employer's plan missed getting in the contribution before that year's end for some reason, or perhaps for some corrective action.