There is no box 1a on a Form 1099-R. Do you mean box 1?
What is the code in box 7 of the Form 1099-R?
Do you have another Form 1099-R from the same account? If so, what is the code in box 7 of that form?
It's possible that the Form 1099-R that you are asking about represents a distribution of nonrecoverable basis in an employer plan because there was a total distribution from the plan that was less than your after-tax basis due to investment losses in the plan. If this is the case and you rolled the total distribution over to a traditional IRA, you might be able to treat this as part of an adjustment to your basis in nondeductible traditional IRA contributions.