Where can I find firmware 7.3.1. for airport express?

Hi there,

Very old airport express needs new firmware 7.3.1. I have a old mac with leopard still on it so I can actually take that hurdle, since I understand you cannot get old firmware like 7.3.1. with mountain lion or higher...anyway - where is the download link? I found this: AirPort Base Station and Time Capsule Firmware Update 7.6.3 but obviously there is no link for download there. And it is .2 higher.... Can someone help me?

Many thanks! Petra