The Kharosthi script (script means "writing, using a particular alphabet") is not used on Rajuvula's coins. But it IS used in his Mathura Lion Capital Inscription, which is certainly official. ('Official' here means Rajuvula authorized it, or commissioned it, I think.)
(Then, in brackets, the source of this information: the author, the title of the article or essay and the journal that published it.)
Later you asked,
The bold sentence i meant. What does "unmistakable stamp" mean here?
Please use a dictionary. You asked about the sentence and I have explained its meaning. Please find out what a sentence is. If you use a dictionary you will understand my explanation.
The word is in the dictionary. It means 'not likely to
be mistaken for anything else'.
Rubber stamps are used to mark official documents. The ink-mark they make on paper is also called a "stamp". These marks (stamps) are always distinctive: each one is recognizable. A document stamped with the stamp of a government department becomes official: it now bears a stamp.
Imagine a smart young man marching along the road. You might say, "He looks like a soldier." Or you might say, "He has (or He bears) the stamp of a soldier". Do you understand? It is a metaphor. He HASN'T got an ink-mark proving that he is a soldier, but we might SAY he has. He looks as if he might.
The Mathura Lion Capital inscription hasn't got an ink-mark proving it is official, but we might SAY it has. We might say, it "bears an unmistakable stamp of official association."