16.505(a)(4) vs 16.505(b)(2)?

If I have a Multiple Award IDIQ where only one vendor has rights to provide a certain brand name product, determined to be vital to the requirement, but in the open market there's several other vendors who have rights to provide the same product would this be subject to just 16.505(a)(4) or also subject to 16.505(b)(2)? My concern is that if it's subject to 16.505(b)(2) I'd consider it under the exception "(B) Only one awardee is capable of providing the supplies or services required at the level of quality required because the supplies or services ordered are unique or highly specialized." However, I'm not sure the bolded part applies if there's other open market vendors who can provide this.