The tiny pin should come out...it essentially only is long enough to come out of one side, acting as a lock to keep it from being 'easily' removed...in the normal total pain in the tookus manner that the same sights without the pin are removed...why they added the pin is beyond me. The first time I encountered one, I didn't realize it had the added pin, and I thoroughly destroyed it. Perhaps some PB Blaster might loosen it....problem is finding a punch small enough to not flare out the ends in the sight leaf.
You might be able to coax the spring back into its niche at the rear of the sight block with a punch and hammer, tapping it back into it's slot, which is about the depth and thickness of a trimmed off thumbnail. I don't know if you can quite get the angle needed with the leaf still in place though.
What helps in reseating those is complete removal, and a good cleaning in the slot, which has the tendency to retain cosmoline and any crud that might stick to cosmoline...(everything).
I didn't know Romania did that too, I thought it was a Chinese 'innovation' only. I have seen how Russian kids are trained to field strip AKs, and the removal of the rear sight is part of the process, in a crazy karate chop manner. I have no idea why they would remove them...but if they encountered the pinned ones, it sure would smart when the pin stopped their hand!
Don't fire it until you get this resolved, I had a loose one vanish once that way....I found it three years later in my folks yard. Now I have a spare.