With some excellent advice from members of the Golfballtypewriter forum, I was able to fit my spring onto the interposer arm. However, I found it was not an exact replacement. It was too long, so that it didn't apply the necessary tension to pull the interposer rearward.
I am working with other springs to see if I find one I have enough confidence in. I also posted a request to the forum to see if anyone has a new-old-stock replacement spring. I can additionally salvage one from another Selectric typewriter, I think. They are relatively easy to buy since most have stopped working due to solidified lubricants.
I made use of one spring, not quite identical but seemed to do the job. I tested it with hand cycling and it now latches each and every time the tab button is pressed. The process of reassembly has begun. The backspace operational latch has a spring that has to fit to the top ear of the backspace interposer - it involves pushing a spring away and looping it over an ear, rather than pulling it toward me.
Fortunately, there is an IBM service tool for this, called a captive hook spring tool, which grips onto the far end of a spring (the circular hook), which allows you to maneuver it onto its holder, after which it can be released from around the spring hook. Thank heavens I bought the specialized tools needed for Selectric work - some of these tasks would be impossible without them.
That spring attached, I could reinstall the mainspring plate, the bracket that holds up the escapement rack at midspan, and set up the various linkages. I have just three things that remain to do before this is reassembled and ready for validation testing.
- Attach the carrier return cord pulley on the mainspring shaft with the cord taut
- Attach the escapement pulley on the far end of the mainstream shaft with its cord tensioned properly by the self-adjuster
- Install and wind the mainspring to its proper tension