Well, I didn't notice this one coming but I'm in. With all the firsts in the world to choose from, I'm surprised how aviation this bg looks and wanted to broaden it out.
I'm going with
https://www.scalemates.com/products/product.php?id=109407&p=timeline
which is The world's first steam locomotive, can't show you it yet as I've only just ordered it! Have to say not my first choice which was going to be the first locomotive to break the 100 mph barrier, City of Truro but Nimrod54 did this so brilliantly that I wouldn't come close, if you missed it here's his build
Hunting around I came across the Airfix kit for Trevithick and a cheaper Minicraft version but that is a strange 1:38 scale while the Airfix is the more conventional 1:32, which is gauge 1 for railway modellers, so it might get a crew to attend to it. But all that to come, first I need the Postie.