Taking apart an enlarging lens

I have an enlarging lens that i would like to clean the glass on the inside, it is simply constructed and it seems the only way in is some sort of ring that holds in the glass from the back element with two small holes on opposite sides that some sort of key goes into to turn it, i have been forcing this thing to turn as hard as i can with no luck, any suggestions?

The tool needed is a spanner wrench. It fits in to the holes/slots and provides adequate leverage for turning the ring. The important thing to remember is how the elements come out and which side is up. If the elements get turned over or out of order you have a big problem. I have cleaned many lenses using this tool. You can get one at Micro Tools fairly cheap.