Loke, As rugerguy stated, something isn't right. Old Model 45 cal Blackhawks were only made in 1971, '72, and '73 and had a serial number prefix of 45. New Models started in 1973 with a s/n prefix of 46. Both OMs and NMs had XR3-RED grip frames. Some OMs were shipped with Dragoon style brass grip frames, identical in shape to a Super Blackhawk.
Simple solution …. if your BH has 3 screws in the side of the frame, it's an Old Model. If it has 2 pins, it's a New Model.
Here's my 4 5/8" OM Blackhawk with a S/N prefix of 45. Note the three screws in the side of the frame: