From The Manufacturer/Wikipedia/People Smarter Than Us:
The 1866 was already quite popular when the smaller, yet quite similar 1873 model was released. It became an immediate favorite. The Winchester Repeating Arms Company may have thought that domestic military sales would make up most of the orders for the firearm, but in fact, civilian sales and international military purchases were what brought in profit that year. In that day, the 1873 was the first lever action available with a broad variety of options. Today, an array of options is still offered on fine reproduction models by Taylor’s & Company.