There is no reason to purchase that old breaker.
The FA in the numer tells you that it is a standard 100A frame, 18kA@480V depending on its age.
The quirk is the letter B, as the third digit. This B was for a bolt-on style connection that was designed to satisfy the specifications that 'prohibited' plug-in connections. I believe that they stopped offering this option several decades ago, as there was not enough market demand. Square D's supplemental Digest used to have pictures of it. The breaker to bus connection was the standard I-Line connection, the B option added a pinch type connector that mad it slightly more difficult to unplug the breaker by accident.
Have your customer purchase a standard I-Line style 2-pole 40A breaker.