MicroSculpt,
You are correct, this is not a typical German practice but the standard procedure before the first flight of the day for the WW1 inline or Vee type engines,German, French or British.
To check the oil level the engine thus the airplane must be level. It is also of upmost importance to clear the cooling circuit from all the air pockets that could create a malfunction or even dammage the system or the radiator itself.
This is a little more complicated to explain, but to make it short, the little tap used to clear the circuit ( SE, SPAD etc...) is situated on top of the carburator and the engine must be level in order for this tap to be the highest point of the circuit before the radiator ( hope this is clear enough....

) We did not made up this procedure, everything is in different technical manuals ... Just read and apply.
And believe it or not, it works !
Regards
Alain