Looked at the 3rd pic again...need to run a tap thru the threads on the elbow where that boogered-up shoulder screw was removed. size is M10 x 1.50. use a plug tap, not a bottoming tap. It's a thru hole but the threads are not fully cut on exit. most tap and die sets only have plug taps where first 3 or 4 threads are tapered. just run the tap thru so that 2-3 tapered threads are sticking out. don't fully cut the threads in the elbow.
when the stock components are assembled with a new donut gasket (Felpro 60755), only the starting thread plus one thread on the shoulder screw emerge from the threaded hole on the elbow.
In pic 3, the boogered-up screw threads on left got rounded over from the remains of the the 8th thread left behind upon removal and/or deformed the rest of the threads upon removal. Threads on the right screw are pointed and have the correct pitch.
If you don't run a tap to chase the threads in the elbow, the replacement shoulder screw will get rounded threads as well.
You may be able to run a die over those damaged threads to correct the rounding over and cut a new 8th thread albeit that 8th thread won't be complete. Only need 4 threads to secure a part. Damage is at 8th thread and the first two threads pass thru the hole. Plus, it's a shoulder screw and you have the spring. Not a lot of torque or sheering forces in play here.
However, as the die cuts those rounded threads, material is removed, so the resulting thread pitch will be less and may cause the screw to seem to wobble in the hole.
might be something to try while you are waiting for the replacement to arrive in your mailbox. you do need to chase the threads in the elbow so that the replacement screw fastens cleanly.