If Harris Holland's serial numbers were sequential, you're probably looking at a muzzle-loader built around 1864 or so, when Holland was also making pinfire guns. That's late-ish for a muzzle-loader, though it also means they were at their peak of design and quality.

It has always been fascinating to me that in 1864 a sportsman had the choice of several popular systems to choose from, from percussion to pinfire, needlefire and centrefire.