The boxes on the 1099-R and the CSA 1099-R are the same, except that the 1099-R has a few extra boxes that never appear on the CSA 1099-R.
That said, the same data in the same boxes will yield the same results on the federal return whether you enter it as a 1099-R or a CSA 1099-R.
I can think of only two differences:
1. If you choose to call it a CSA 1099-R (by checking the box after adding it), then the OPM information is automatically included, which saves you a small amount of typing.
2. If you live in a state that taxes normal retirement distributions but does not tax federal government retirement distributions (or, at least, taxes them differently), then if you use the 1099-R instead of the CSA 1099-R, the state might not automatically handle your retirement distribution correctly. But every state does it differently and you probably have an override to remove the federal pension amount anyway - I can't say without more information.
Outside of this, I don't think it matters. However, to be sure, you would have to run the return both ways (as a 1099-R and as a CSA 1099-R). But if you live in a state with no state income tax or which taxes all retirement the same way, you shouldn't even bother. Just enter it on the 1099-R on the principal that you enter what you have.
