That "shallow trough (groove) on the upper inner frame @ the rear of the bbl" is what's known as flame cutting.
You're leading is coming from what's called gas blow-by. You're using a light load of powder and it isn't expanding the base of the bullets enough to seal the bullet/cylinder fit and gas is escaping around the base of the bullet flame cutting as it goes. Then the gas with the flame cut lead is depositing lead everywhere it goes.
It's pretty common advice that you'll lose +/- 10fps for every 1/1000th's of cylinder gap and when using 38spl's in a 357 cylinder.
You're not getting leading in the bbl because you have the correct bullet/bbl fit. In the cylinders, not so much. You're problem is the bullet/cylinder fit. (.357" bullet)
Some simple fixes:
Use a larger bullet diameter
Use a softer alloy
Use more pressure/powder
Move the bullet out further/longer oal
Take a couple bullets you're loading and drop them in your cylinders. Note where the bullet noses end/sit in the face of the cylinders. That is where you want to seat/load/oal your reloads.
I love bullets that have 2 crimp grooves!!! The Mihec 640 for the 38spl's/357's has become my favorite bullet to use when reloading the 38spl's.
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Doesn't matter if I use the reloads in any of my 38spl's or 357's. The bullet always gets crimped in the bottom crimp groove. I also like to move the wc bullets out further/longer oal when using them with the 38spl reloads/357 firearms. I crimp the h&g #50's in the middle lube groove.
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Those reloads pictured above produced these targets. The targets are not hand picked/cherry picked targets by any means. That are nothing more than the actual test targets I used to tests loads for a 686. 6-shot groups @ 50ft testing 38spl loads for a 686.
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What the 686 looks like after a 200 round range session with those reloads pictured above.
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As you can see in the picture above, that 686 has a buildup starting to form in the same area's that you pictured in your post. The difference is my bullets/reloads only have carbon building up on the firearm. Your's has carbon & lead. The bullets/loads that I use are:
8bhn/9bhn
sized to .358"
+/- 15,5000 psi
The other thing to note is in the 200 round range session picture posted above you don't see any soot on 1 side of the case mouths. Seeing soot on 1 side of the case mouths ='s the pressure of the load is too low to get the brass to expand/seal the cylinders. If you see blow-by on your cases you'll see blow-by on your firearm.