I believe that one of the differences is the thread pitch on the injector bodies (and, similarly, the prechambers). I don't think that I took any pictures of the injector when I pulled it out, but I believe there are parts numbers stamped on the circumference of some surfaces.

A pretty crude way to check would be to mark the injectors with a piece of tape (which is which), and then compare side by side with a decent pair of calipers (if they're not obviously different). I would just be quite careful not to scratch anything with the calipers. My Mitutoyo set are very hard, and very sharp...

Is it as simple as swapping the prechambers (or possible?)? If the two heads are different by the angles of which the injectors sit at, among other things, it seems like the glow plugs may interfere with the different geometry.

I apologize for the amount conjecture...

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting