Marshal Van Cleef says that his authority is vested in him by "the Territory of Nevada". The movie is set in the early 1880s, but Nevada had become a state some 20 years before, in 1864.
how is this a slip up? you even said that the thing said "territory of nevada" not state of navada. they had desigated places in the US named territories that had the names of our modern day states. not a slip up.