Those brown disc capacitors are fairly modern. They are marked with a value either in microfarads or picofarads with a multiplier e.g. 104 = 10 + 4 zeroes = 100000 pF = 100nF = 0.1uF. This is similar to resistor codes. Very small capacitors under 1000 picofarads do not use this system. Instead, the actual value, in picofarads, is used, such as 10 (for 10 pF). So the disc capacitor marked 20 KN is 20pF.
What about the other letters? The letter after the numeric value is usually the tolerance. J=5%, K=10% and M=20% so in your examples
0.01M = 10nF +/-20%
0.05uF KCK = 50nF +/-10%
20 KN = 20pF +/-10%
Working voltage? This is a hard one to answer. Some manufacturers add extra letters or symbols to represent the working voltage as well as other characteristics like temperature stability. I'm not sure if there is a standard for this. Sometimes the working voltage is written directly (e.g. 250V or 1KV with an AC or DC symbol after it) in which case there is no doubt.
If there is no working voltage obviously marked, I would assume them to be low voltage (say, less than 50v). Those brown disc capacitors are commonly found in low voltage consumer electronics, like Made in Hong Kong pocket transistor radios which run on 9 volt batteries. I don't think they will be high voltage capacitors unless clearly marked as such. I've found the red tubular ones in a Vidor battery valve radio (90v H.T.) so those may be good for slightly higher voltages.