1907: "Fortpflantzungsgeschwindigkeit" ;)I would normally take that term as more general than just referring to one-way light speed. I understand the context better from your latest post. (I do wonder what the original word Einstein used in German was that got translated as "propagation".)
For me that discussion is pointless (or at least, exhausted). As we know, Einstein assumed in 1905 that - once more: "from experience"! - the two-way speed of light is a universal constant (the speed of light in vacuum), or in German: "die Lichtgeschwindigkeit im leeren Raume".
[edit:] I'll do one last attempt to explain this, as this "bug" keeps on returning...
The 1-way speed of light rays in all directions can only be made a constant by means of appropriate clock adjustments if the 2-way speed of light rays in all directions is a constant (not by definition, but by measurement!).
If that does not help, I give up. Perhaps Peter can explain it clearer!
A nice example of the constancy of the speed of light (Einstein could not cite it in 1905/1907 as it had not yet been performed) is the Sagnac experiment. No matter how fast the light source and mirrors move, the propagation speed is unaffected ( in vacuum, and practically in air; it's slightly more complex with a glass fibre). Consequently light can be used to measure the speed of rotation.
- for elaboration see https://en.wikipedia.org/wiki/Sagnac_effect ; regretfully the current version has "invariance" which could be confusing as only a single inertial frame is concerned ("constancy" would be clearer in this context).