I am the person who took the picture. I am pretty sure it's not a 7.7 cm FK 16, the gun in Sofia has a much shorter barrel and a multitude of details just don't match.
Charlie, I am afraid you are basing your assumption only on the breech's markings that say Krupp and 1916.
I have not identified it yet, but it is not a 7,7cm lFK 1916.There are no axel tree seats, the shield does not over-hang the wheels, and is straight without the slight curve of the 7,7cm lFK 16.The second band around the barrel is too small in comparison to the 7,7cm lFK 16 too.See an example of a 7,7cm lFK 16 in my collection for comparison:
Perhaps the following comment from Osman Levend in the Ottoman section of the Axishistory forum might suggest what the "approximately FK 16" is:
"I know for a fact from my late Father (Osman's father was an artillery officer during the War of Independence) that some of the minor parts were sent to Istanbul when damaged to be repaired or to be re-tooled from scratch at the old Cannon Foundry (Top Hane i Amire in Istanbul) and were carefully marked so that there was no mix up when sent back to the front. He told me that even during the Republican era, it was a standing order or a custom acquired from previous wars that if an artillery piece broke or was damaged; every single little part was carefully collected and marked according to make and type so it could be re-manufactured even if necessary drawings were missing. It might sound a bit odd that such a precaution was needed since Krupp and other major manufacturers always supplied such vital info when they shipped their guns but you must consider the fact that the Ottoman Army also used a variety of captured guns from the Russians, the French, the British etc."
Osman also made the point in another post that Top Hane would machine down barrels which had muzzle damage and return repaired (and shortened) guns to service - the troops apparently were not impressed by this since their firing tables no longer applied to the repaired guns.
Perhaps the gun in Sofia Museum started out as an FK 16, was damaged and then remanufactured locally.