Type K Matt Fresnel field with 3mm-diameter. BriteView split-image rangefinder spot surrounded by 1mm wide microprism doughnut, Rapid, accurate focusing for subjects with both straight and ill-defined contours. For general photography.
Will this screen fit with an early 80's FM?
It will fit, but depending on the model you might have to compensate for exposure.
for full details on focus screens for Nikon FM Cameras google; "focus screens for nikon fm" and it should be the first link "Nikon FE/FM/FA Series - Focusing Screens"
Could I put this screen in my D5100?
I do not believe you can. The tab on the screen for D5100 is located on the side of the screen not the front.
You: Dose any one know for sure if metering if affected and by how much? I know if you put a K screen instead of K2 its -1/3 but what about K2 to K3 replacement?
it depends on what nikon body you are putting it in. for the F3MA and FM2/n it requires no adjustments.
google "Nikon FE/FM/FA Series - Focusing Screens" for a page with specific details