Except in the case of a distribution to multiple beneficiaries of an inherited 401(k), there should be no entry in box 9a for a distribution from a 401(k). Leave it blank. Enter the Form 1099-R exactly as received.
You didn't mention it, but there is likely code G in box 7.
Presumably this is being treated as a direct rollover from the original plan back to itself. That's a bit unusual, but probably will not raise any issues.
