This is your tension bolt:
https://www.niagaracycle.com/categor...-bmp173-chrome
If your saddle has stretched out you may want a longer tension bolt:
https://www.niagaracycle.com/categor...-70mm-with-nut
You may also have lost the shackle clip:
https://www.niagaracycle.com/categor...17-narrow-flye
But it may be very difficult to get the tension bolt into the saddle without damaging the leather. The steel frame is rather springy, so what you need to do is attach the shackle clip and bolt, then compress the frame (push the front toward the back) until you can slip the end of the bolt into the hole in the nose piece. It is really hard to do. Really hard. Don't try to stretch the leather; it will tear before it stretches.
If you can't get the tension bolt in by compressing the frame, you'll have to remove all six rivets from the cantle plate, put the frame together with the rivets out, then re-rivet. You don't have to use Brooks rivets; 8-6 or 7-6 brake lining rivets will work.