I have a 7400 rear derailer. The barrel adjuster is one piece - no plastic housing or spring or ridged plastic washer. I have a broken Suntour Edge rear derailer and its barrel adjuster is also one piece and looks compatible. It's the exact same length as the 7400 barrel adjuster and it threads into a Shimano 600 rear derailer I have which is the same vintage as 7400 (the 600 barrel adjuster is very likely thread compatible but it is longer to allow a ridged plastic washer and spring to be inserted).
Since the Edge derailer is shot (I was saving it for parts), you can have its barrel adjuster if you can't find anything where you are. PM me with your address if you want it.