We added ours after purchase. It was an option, but none that we wanted had them, and I didn't feel like waiting 8 weeks this go around. Even though it was an option, we contacted the factory, spoke with a few people and verified it was blocked / backed to accept the brackets on the outer slide wall corners.
I'm sure it's ok, but I'd verify with the factory first that it is indeed blocked to accept the screws where they need to be.
John and Rebecca Dickson
Emma-8 / Little John-5 / Iva-1+
2013 Ford F-350 Lariat FX4, CC LB PSD, DRW
2015 Jayco Jay Flight 28BHBE (#8)