It's actually the small rectangular piece that the hammer strut passes through (#28 on the schematic you referred to). So, you have the hammer strut that has somewhat of a two pronged fork on one end. This presses against a circular depression at the base of the hammer (when installed), and then the spring goes over the strut. The strut bridge then slides over the strut. The strut is, for the most part a rectangular tang of metal approx 2 inches long and an <1/8 wide. So, the strut bridge has a small rectangular hole that the strut passes through. It seems obvious that the strut bridge serves the dual purpose of holding the otherwise loose end of the strut in place, and also to provide a stop for the spring to compress against. The real mystery is what holds the strut bridge in place. There is no obvious "slot" or any type of stop that would serve this purpose. The only thing that I have been able to assume is that the buffer will actually provide the "stop" for the bridge. But, since I don't have a buffer at the moment, I can't verify.
Thanks iin advance for any help.