Im talking as a repairman now.

There is a possibility that the former owner did put new screws in that were wider/thicker than the original ones.
It is (verry much) posible that there are 2 slits in the barrel that should move.
If that is the case you will need to file down the thickness of the two screws at the end part of the screw, just the thread, so they will fit into the two sits without locking the front barrel up.

I wish I had the lens in my hands in Holland, then the problem would be solved in an hour or so, maybe less.