However, the answer was dependent on the presence in this location of a certain trade or industry that I knew it engaged it at more or less the period in question. Finding the actual dates was really, really hard, but after several hours of searching, I've now discovered that this trade or industry didn't start up in this place until about 20 years after my story is set.
So: Grr!!!! I know I could just ignore the pesky fact of historical reality and include this thing 20 years early, but... well, I can't. Now that I know it's wrong, I can't do it. It's like when I discover that the pivotal moment in a plot is contrary to the laws of physics, or when I unearth an obscure canon point that contradicts the entire premise of a story I'm planning. Don't you just hate it when reality interferes with a story? Grr!