The "whole" conch that GB shows looks like it's been opened with a machete, which is an historical method. There are heaps of "modern" conches with that exact same slash all over the keys and islands, it's where the animal attaches inside the shell. The aboriginals just bashed the shell open or cooked it open, but they often bashed it in the same spot to release the animal.
Great, HEAVY find there, GB!