There are various springs that press against either a ball or slide valve, kind of hard to tell from that picture. If you have a shop manual, look at the valve body section. I can upload pictures from a '70 manual if you need.
I just put a valve body shift kit in my 727, and don't recall seeing anything that small in there. Looks like its too short to fit anywhere. Is your shift lever detent ball and spring in place and working properly?