We have developed, manufactured, and characterized a WM-570 rectangular waveguide bandpass filter with a central frequency of 415 GHz. Initially, a vector network analyzer (VNA) with millimeter-wave converters was employed to determine the transfer function of the filter within the WM-570 band of 330-500 GHz. Subsequently, we utilized a frequency-comb locked continuous-wave terahertz spectrometer to cover a frequency range from 300 to 800 GHz, spanning several waveguide bands. Using this system, in addition to the designed passband around 415 GHz, we identified several other features at higher frequencies, particularly beyond the measurement range of the VNA converter setup employed. The measurements agree well with the simulation, even at frequencies beyond the operational bandwidth of the WM-570 waveguide. This demonstrates the potential of a tunable optical source to characterize measurement instrumentation up to THz frequencies.