I think I've got it... The 722.1 ATSG transmission service manual I have was of no help (as usual), but if you look closely at this picture, you can see how the spring goes.
The 90 degree angled end of the spring goes in a hole, which can be seen in your picture just to the left of the parking pawl, in the middle of it.
The other, squared off end of the spring goes against the right side of the parking pawl, but the spring needs to bend in on itself so it pulls the pawl away from the cog in which it engages. I think the best way to do this is to stick the 90 degree angled end of the spring in the hole in the transmission case, then set the parking pawl on the pin about 180 degrees from the resting position so the boxed end of the spring rests against it, and then rotate the parking pawl clockwise against the spring pressure until it's in proper position, the push it further down until the metal bracket on the left side of it holds it from springing back.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles