Ottoman East Rumelia Expertising Marks?

I bought 2 lots of Ottoman stamps awhile back. I can't seem to resist all the history in them. The Duloz overprints have made me crazy enough but then I found this nice stamp. It's (if I'm not mistaken) Scott nr. 17.
On the back there is an A superimposd with an H (or is it vice versa?). Is this an expertising mark? If so, how do I go about finding out who the expertiser is/was? Does it change anything in the value of the stamp?

A larger more detailed scan would probably help.

Sorry for the quality of the scan. I don't own a scanner so I had to get a friend of mine who does
to scan it for me. I'll try to do better.

I doubt it is an expertsing mark, it looks like an owner's mark to me,

Okka