Did you instruct the 401(k) plan administrator that this was a Roth conversion of did you decide to put it in a Roth after the check was issued?
Assuming that it was a check and not an electronic transfer, the check should have been payable to your Traditional IRA account if the box 2a is zero. If payable to your Roth IRA then box 2a should be the same as box 1.
**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.**
