Which is where your confusion lies about "constantly changing rpm". If the setting was only allowed for one minute, they you reduce pitch to reach 2600 rpm and coarsen it as speed increases to maintain 2600rpm for ONE MINUTE or a SHORT TIME!!
Given that we know how a variable pitch propeller is operated and the physics of how it transfers power....
Since propellers are optimal at a specific speed and rpm....
We know what the RLM meant in the their instructions and what Ulrich Steinhilper is telling us both fit together without contradiction!!
It is a fact that you operate a selectable pitch propeller by managing pitch to maitain the a constant rpm at a desired setting....in this case 2600U/min.
Round and round the mulberry bush, mulberry bush....
round and round