1967 C40 Rear Axle Rating?

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Good evening everyone, I am working on a 1967 Chevrolet C40, and I need to replace a bad power brake booster, which is mounted under the cab. They want to know what rating the Rear Axle is in it, so I need your help. It is a 1967 C40, L6 292 engine, single rear axle with duals. The tag inside the drivers door jam says the truck is rated 16,000 lb GVWR. The tag that is inside the glove box says it is a Model CS41403, and I also see that it says "RR Spring 15,000 lb" on the tag, but I wasn't sure if this was refering to the Rear Axle rating or not, as I wasnt sure what the "RR" stands for. Can someone help me? What is the rating of the Rear Axle in this truck? Thank You

Brian