What I've heard is that some parts are interchangable, the needle is different, from what I can see, but have never opened up a D shaped speedo yet to confirm anything.
Even though they look the same at first glance they are different. The stabilizer and recorder wheels sit in a different position. http://www.hells-confetti.com/Technical%20data/Jaeger%20Instruments/Jaeger%20Chronometric%20Overhaul.pdf