Thanks, Hans & Kuba. Both methods are solutions but not better than mine in my second post above

sdf = Append[sd[[1]], sd[[2]]]

Somewhat crazy.

The "Interpreter" reference documentation shows many examples, where Interpreter is resolved to what is expected. For example:

In[1]:= Interpreter["Number"]["4.5e7"]
Out[1]= 4.5*10^7

Hence I would expect

Interpreter["FileName"][sd]

to be expanded into the full filename. But it isn't. It gives a RestrictionFailure instead.

Anyway, I can live with my strange solution.

I have to apologize to you, Kuba. It was nonsense to say that your solution is not better than mine.

In fact, my solution is wrong and yours normally works. Thanks a lot. I had tried my solution with a stylesheet from a subdirectory Wolfram\ProgrammingLabDefault.nb only.

We still have a problem, when we add any own additional StyleDefinitions. As an example:

setPS1[] := SetOptions[
  EvaluationNotebook[],
  StyleDefinitions -> Notebook[
    {
     (* Style Environment Names with correct Magnifications *)
     Cell[StyleData[All, "Printout"], Magnification -> 1.0],
     Cell[StyleData[All, "Working"] , Magnification -> 1.0 ]
     }
    ]
  ]

If we execute that once, the findStyleSheet-function will no longer work, since CurrentValue[thisNb,StyleDefinitions] does no longer give the StyleDefinitions for the stylesheet name, but that own StyleDefinitions prepended.

I think I can easily solve that circumstance, but I have no time at the moment.