Try pulling out the old pin and describe it to him, measure it. Maybe buy both on the pre-texted that you will return the one that doesn't fit (agree on this with the supplier first to avoid any problems). Do some research and see if you can determine the age by referencing the serial number to determine which model it is.
Most important I've found that when tearing down anything with small parts or springs, place the pistol inside a large plastic bag. Break it down inside the bag. Too many times I've chased an errant spring that surprisingly shot across the room. When you slide the plate that holds the firing pin inside the slide the firing pin will be under tension be careful as it can shoot out the back and you could lose the spring or worse you could catch it in the eye.
Good luck and be careful.
Si vis parcem, para bellum