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PS - Congrats on the house but don't let your homeowners insurance co know you got a "bad" breed dog. They will probably ask you if you got any pets, they did me and wanted to know what breeds they were. I can only assume it was for liability purposes.
Yup. Don't register it, and truly convince yourself that it's part black lab. We were going to switch homeowner's insurance once upon a time when our Rott was still alive, and we got DQ'd over the dog breed. When I did the back to college thing more recently, I had a prof who was all bummed out because she was running into the same problem. She loved her new dog, adopted from rescue, no papers, etc, and she was running into problems trying to find insurance for a house she was buying because she was identifying it as a rottweiler. She was showing us pictures.
"I think that looks like it has some black lab in it."
"No, no, it's a rott."
"Nope, I'm pretty sure that's a lab mix. You can tell the next insurance company that you adopted a rescue dog, and someone who knows a lot about dogs told you that it's a black lab mix. Now think about the potential benefits of accepting what I said, even if you decide I'm wrong in a few months when the dog is older."