Alexander, many thanks, I am still struggling with his original German name.

It seems to be a combination of the family names Hermann and Fersen, the "von" indicates nobility.

"Hermann" being a very popular German Christian name, makes it difficult to cut through all the Google books results, but it seems a noble family with the double name Hermann and Fersen/Versen is nowhere mentioned. Was he perhaps nobilitated in Russia ?

I found the bio below, but couldn't read it

Oli