I would expect a Jaeger drive type with steel dial and yellow line...As to the type, is your KM20 British WD, Overseas military, or civilian ? This will all have a bearing on the type of instrument fitted.
Pre-war civilian would be illuminated for most markets and probably have a trip.
Marcus, My apologies for not making the link between you and your M20.
The correct speedo for a 1939 WD contract M20 is the 80mph non-illuminated / non-trip type, with the yellow 30mph marker.
It should have the Jaeger type cable connection (where the inner cable is retained by a small grub screw rather than a screw-on outer cable).
This 1939 example attached to Norton bracket was acquired by Jan in Northern France. I can't see if yours has the 'Made in England' and 'Eng(lish) Jaeger Patent' lettering. Apart from that, the only obvious difference is the bright chromed rather than dull chromed WD bezel.
Very few (if any) traders are now selling Chronometrics outright and the going rate for a restored exchange example is around £120. Restorable donor instruments seem to be worth up to £100.
If this one is working and just needs a clean then the lower end of your price range is not too bad but if it needs restoration then it will be expensive. Dull chromed bezels are not available (but the chrome comes off easily enough) and dials can be swapped over.
If you're looking for authenticity then you do need the Jaeger drive so that you can use the thick cable.
Since it has the right face, it is quite likely to be the right speedo drive on the bottom. There should be a sindple protruding from the bottom (no screw thread). The inner cable has a sleeve that fits over the spindle and which is locked in place by a small screw. The outcable has a sleeve as well that covers the spindle drive in the speedo and has clips which fixes it to the speedo head.
Here is the Jaeger spigot cable fitting that Leon speaks of. When I had this speedo overhauled by David Wood for my Enfield WD/L (he had to also reverse the rotation) I got him to loosely fit a bright chrome bezel which I then lightly grit blasted to make it dull. Ron
That isn't the correct cable drive system for your WDM20 - it has a later movement. WD contract bikes didn't have illumination (but it's easy enough to block the hole). Civilians paid extra for a trip function so some have them and others not.
I would see this one only in terms of a donor instrument for reconditioning if you're looking for accuracy...and it seems too expensive for use as an exchange item.