Belated update. Due to a violent border war with my garage, all tinkering was suspended for the past two weeks.
I can say that the struggle was successful and I reestablished territorial dominance of the workbench. I will admit that "cleansing" operations, while brutal, eliminated the resident hoard that occupied the floor of the workshop. A series of purges and reeducation camps for the remaining tools has improved production.
In the interim, the trigger guard showed up, so a few hours before I started cooking Thanksgiving, I welded the damaged dovetails. Today, I made the lever release latch and stud, fitted the sights and put the new piston seal together.
I was surprised to find that the mainspring is an original BSA, as it has a trace of the stacked rifle stamp on it.
I lubed the piston and spring, proceeded to struggle to get it back together, but finally got it done.
The great news is that it has a wonderful shot cycle, very smooth and absolutely no twang with fairly good power considering I was running a very heavy pellet.
The okay news is that the trigger pull is in need of some help and I'm not sure how much adjustment can be done without compromising safety.
The crummy news is that the bore has a section about 4" long that is pitted and rough. No amount of scrubbing, JB bore paste and even Evapo-rust didn't make that go away. As seen by the 15yd target, it's not that great. Now it may behave differently with normal weight pellets, but my hastily set up and dismantled test range is not a safe thing for prolonged testing, so I ran what was at hand without digging around. I also shot my other Light to compare and it seemed fine with the same pellets. My groups are nothing stellar, just quickly snapped off while staying out of sight from the neighbors on the hillside above me. These were offhand unrested and shot between the branches of an orange tree

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So for now, other than some touch up and a bit of bluing, I'm done with this for now. I know it would be more grief than it's probably worth, but I might consider putting a new barrel on it. It's not in any kind of collectable condition, so I probably can't do anything too heretical. I just love the balance and way it shoulders..I'll need to take the other Light apart and see how the piston looks. That one has an annoying shot cycle with a twang that rings my ears. The power is almost as much as this one I just put the seal in, but I'll bet it could use a bit of freshening up.
I hope you all enjoyed this fiasco.
Mark