Well, keep in mind Montana didn't become a state until 13 years later in 1889, and the county that Philipsburg sits in (Granite County) wasn't even established until 4 years after that in 1893, so records are sketchy, because simply no one was required to keep any. Philipsburg has had several cycles of boom and bust, so population rose and fell accordingly. What records we do have all come from mining companies.
Here's what we know as fact.
In 1866 as the Civil War ended returning veterans established two mines in the area, the Hope mine and the Commanche mine. The mines became attractive financially and so In 1866 the Hope and Comanche mines were obtained by the St. Louis & Montana Mining Company. They sent a guy there to set up the smelter, mill and mining processes, a 34 year old mining engineer named Philip Deidesheimer. When a camp grew up near the Hope mill, it was named Philipsburg after the man in charge of the mill. The standard joke was that Deidesheimerburg was just too much of a name to handle. The camp grew and became the town of Philipsburg, officially originated in July of 1867 and continued to grow at a rapid rate. One observer estimated a new house appeared in Philipsburg every day. By the end of the year of 1867 250 houses sheltered a population of 1,500 (so that's 6 people to a house). Businesses included 6 general stores, 7 saloons, 3 blacksmith shops, 2 breweries, and 3 livery stables, a steam saw mill, a hotel (Dana's) and a newspaper. Services were provided by 2 doctors. A Masonic hall, the equal to that in Helena, was erected at a cost of $6,000.
The Hope mine had its own mill called the Hope Mill which used a new process to extract silver. This process required large quantities of salt in order to work. The Hope mill ran until 1869, but the process was not effective and much of the silver values escaped to the tailings. Then, because salt had to be fetched from Utah at a cost of $120 per ton, the mill had to shut down when the cost of salt rose to $320 per ton. By 1869, only 3 people remained. However other people came and other mines opened and were called Poorman's Joy, Speckled Trout, Kitty Clyde and the Pocahontas mines. In 1870 another mining company came in and started leasing some of the mines and mills, and the town started to revive. This worked until 1873, when we had the Great Financial Panic of 1873, and things went bust again for awhile. A $150,000 mill was built in 1875 however a mile east of Philipsburg which employed a newer and better milling process. A small camp of 20 cabins was established adjacent to the mill. Cabins are smaller than houses, so let's assume 4 people to a cabin, which would make it a population of 80 people. This camp was at first called Troutville, but later was known as Tower after C. Tower who controlled the operation. Other mines in the area included the Salmon, Gem, the Little Emma and the Osage. There was at least one other mill to support them, so let's say another 80 people there. Add in other business support folks blacksmith, saloon keepers, etc, and best guess for 1876? Let's say around 180 to 200 people, but growing.
Today, the population is 921.
There was a book published in 1963 by the way, called Montana Pay Dirt, written by Muriel Sibell Wolle, which has a lot of the details you might be interested in that covers this area at this time period.
