Correct bolt knob material on a K11 is deep red composition of a kind of resinous/plastic-y nature. Leastways, that's what's on mine.
Other handy tints and hips to tell them apart -
1. The K31 magazine butts right up against the trigger guard - as you can see from the OP's pics, the K11 has a good three-inch gap between them.
2. The K31 action is longer because the bolt is almost two inches shorter than the K11.
3. The K31 sight radius is correspondingly longer because of the above.
4. The K31 has a smooth receiver - not grooved like the K11.
5. The K31 has a short length of barrel protruding from the sight block, rather than the long item of the K11.
There are, of course many other differences, but that should get you started.
tac