TurboTax uses an input screen based on the 1099-R because the boxes that exist on the CSA 1099-R are the same as the larger 1099-R form.
This means that you may see some entry boxes on the input screen that don't correspond to anything on the CSA 1099-R.
Don't worry about it; enter only what you have on the CSA 1099-R and ignore any boxes on the screen that don't have a corresponding box on the CSA 1099-R. Just use the 7a box in your case and ignore the 7b box.
For box 7, you should choose "2-Early Distribution (except Roth), exception applies", because that's what it is.
For whatever reason, the OPM changes the wording to "2-NONDISABILITY", but a code 2 is a code 2.
