Unknown Countermark On Gold Ottoman Coin

It is a gold-seller's mark, indicating that this is not a genuine coin, but rather, a "jeweller's copy". It's still made of gold ,and thus worth its bullion content, but is of lesser interest to collectors since it is not a "real coin".

In the world-famous gold market of Dubai, selling fake/replica/imitation gold coins is legal, so long as the replica is made of actual gold and the seller has clearly marked the gold fineness on the replica. In this case, the fineness part of the countermark is on the left: either "21" or "22", written in Arabic numerals, indicating 21 or 22 karat gold. The two symbols on the right are letters, presumably the initials of the seller - looks like it might be the Arabic letters "K F", but I'm not sure.

These gold-sellers make their profit by selling "coins" that are either slightly underweight, or slightly less fine than the genuine coin. In this case, it depends on whether that fineness-mark is a "21" or a "22" - that second digit is kind of indistinct. I suspect it's a "21", making the coin slightly less fine: the genuine 500 kurush coins are 22 karat gold. But check the weight; it might be 22 k gold but slightly underweight.