Recently, a square-pinhole collimator has been proposed for SPECT imaging since it can tile the projection onto the detector efficiently and the fields of view (FOVs) in transverse and axial directions become separable. In this work, we have conducted sensitivity and FOV experiments for eighteen fabricated platinum/iridium(Pt/Ir) inserts with sub-millimeter square-apertures. A sin qθ fit to the experimental sensitivity has been performed for these inserts. The mean (standard deviation) of fitted sensitivity exponents (q) and effective diameters (d) are 10.7 (1.3) and 0.35 mm (0.02 mm), which are close to the values obtained from the simulated sensitivity data generated using the geometric parameters of the designed inserts. From projections of the FOV on the detector, we have measured the acceptance angles in the transverse and axial directions. The measured mean (standard deviation) of transverse and axial half-acceptances are 17.33° (0.21°) and 15.03°(0.23°), which are in good agreement with the designed values (17.13° and 14.93°). These results show that the physical properties of the fabricated inserts with sub-millimeter aperture size match our design well.