I am a huge fan of Leopold keyboards in general for many reasons. I cannot think of a bad thing to say. One of the reasons of course is the quality keycaps- they are thick PBT with a nice, gritty texture and clear legends. I own two models- the FC980M as well as the FC660M. I happened to noticed that the keycaps just felt and sounded a little different and I was liking the keycaps on the FC660M just a little more. I took them off and compared them. They are both double shot. However, when measuring with calipers the caps on the FC980M measured 1.4mm and the FC660M measured 1.55mm.
I did not compare them side by side as one board is at work and one is at home, however as I'm will be purchasing more Leopold in the future I'd like to know more about this. Was the thickness changed at some point in time over the whole line, do these models use different keycaps, or is there some other reason for this discrepancy?