I'm trying to work out if there is an alternative starting point for the second postulate of special relativity. My main observation is that all "clocks" are, internally, based on light signals. So all clocks can essentially be thought of as light-mirror-clocks (I won't expand on this idea much, but it is motivated by the fact that the time difference measured by light-mirror-clocks is independent of the orientation of the light mirror, i.e. independent of whether the light-mirror-clock is up-down or left-right).
Given this, what we think of as time is just the number of bounces of light inside clocks.
In a similar manner, an apparatus for measuring the speed of light is also essentially just a light-mirror-clock (with some known height h). In which case, we can think of the measuring apparatus as being essentially identical to the clock, except that clocks are generally regarded as smaller, say with height h/N where N is some integer for convenience.
Now imagine two moving laboratories moving to the right, lab A is moving faster than lab B (hence the angle theta is smaller than the angle phi in the diagram): Each lab has its own clock, whose height is smaller than the apparatus by the factor N.
When A's apparatus has completed one bounce, A's clock has completed N bounces. Therefore lab A concludes the speed of light is distance moved/time = h/N.
When B's apparatus has completed one bounce, B's clock has completed N bounces. Therefore lab B also concludes the speed of light is distance moved/time = h/N.
So both laboratories measure the same speed for light, and so will all laboratories. This is because the clocks they use always mimick the apparatus.
Note that this result is independent of the "actual" speed of the rays in the sense that I did not need to use a velocity in the above calculations. If the rays "really" move faster in the apparatus they will also move faster in the clock, but the measured speed is the same.
So, can we say all observers should be expected to measure the same speed for light because all observer's clocks are based on the same light signals they are trying to measure?