Belize has several species of conchs, including the queen (the one you eat), the milk, hawk wing, and fighting. Off season, you may get a conch dish, but it is the horse conch (not a true conch).
There are lots of variations in the local naming of things, thus the reason for taxonomy (and I guess the reason I had to learn those damn genus and species names for all those marine creatures I studied).
As far as I know, there is not a common conch so this could have been a name used either by a certain region or may have been the name used prior to the proper classification of the conch.
When I was in the Bahamas a while back, I asked my aunt about the abundance of queen conchs and she said that they were so depleted that they now either harvest the trumpet triton (but I don't know if it makes a pearl) or they buy their conch from Belize!