If box 2b Taxable amount not determined is not marked, the payer really should have put in box 5 the same amount as is in box 1. However, TurboTax will go by the amount shown in box 2a as the taxable amount. (I assume that there is nothing in box 6; it's impossible for box 6 to equal box 1.)
The reason that it's more simple that an IRA conversion is that there is no reporting of this on Form 8606. Form 8606 has nothing to do with distributions from a 401(k).
