I believe you should be able to order the replacement core by the CAT part number or the application depending on the company that you get one from.
To my knowledge there are only 2 options on the U series D2's........ with oil cooler and without.
When you're getting a new core is a great time to remove the original oil cooler and loop the oil cooler lines together just in front of the oil filter.
Or if you want the authentic look, get the narrow core, leave the oil cooler in and hope it lasts a while longer.