I don't remember the name, but there's some Intel utility that is supposed to identify exactly what CPU is installed. If I remember correctly it was designed to identify the intended speed of the CPU, regardless of how it is clocked, so as to identify CPUs that had been overclocked. I think it was intended to expose retail shops that sold overclocked systems (something Intel obviously wanted to stop). I don't know if it's able to detect this info on Pentium MMX chips though, or if it only works on later chips.
Have you ever run that utility with the P266 MMX? I think it's included on the "Ultimate Boot CD", if you have that. My memory of the utility is vague, I haven't used it in a long time.
I like how the case for your P200MMX has a 3 digit speed LED. I assume the leading digit doesn't go above "1", but still, it's twice the range of other displays.
I think it would be really cool to have those speed readouts back on modern cases, and be able to signal them from the motherboard to match all the switching P-states on modern CPUs.