A number board screwed into the main bulkhead - easily removed. And in the lazarette - plastic numbers glued on fiberglass and epoxied over.

We have been boarded bu the USCG many times. They have checked the HIN, seen the number board on the bulkhead and have never been asked or challenged about it's permanence. The epoxied stick on numbers were the recommendation of the agent that did the documentation when I first purchased the boat. It had been registered in the Cayman's to avoid taxes so the number was newly issued.