[Phantasm]

I tend to start TL6 at 1870 or even 1865 when it comes to weaponry myself.

That said, calling the era between the American Civil War and the Spanish-American War its own TL (for argument's sake, let's call it TL5.5) isn't a bad idea, per se. It has its own aesthetic distinct from the Colonial period of TL5 and the WWI-Roaring Twenties period of TL6. Quite a bit was in flux in that time period, as the internal combustion engine was not yet replacing the steam engine, and was actually competing with the electric engine for dominance in the horseless carriage market (which were primarily the rich folks).