Saving my DA*50-135/2.8

A while ago I had to put my belowed SMC Pentax-DA* 50-135mm f2.8 on the shelf. When unpacking it from my back bag I noticed an unpleasent noise. It rattled like glas against glas! I was sure I hadn't handled the lens or the bag careless, but others might. Visual inspection revealed that a lens element was loose inside the lens, and with it, what looked like a rubber or paper ring. On the shelf until I know what to do... This lens is one of the SMD lenses where some have reported that the in-camera-driven focus have froozen. But that was not the problem here.

Removing the first double element was easy (don't try this yourself unless you have the right tools). The third element was a bit trickier, but it worked. This exposed the loose fourth element. Inspection showed it to be OK: no marks on the glas or coating. The ring I had seen through the glass was of paper, it seamed. Unfortunately it had been torn...probably because I had been zooming or focusing before I noticed the damage (the last shots with the lens are optically very strange). I removed also the next element to check if it was damaged, but not from what I could see.

How to fix this? The top three elements were all fixed with two threaded rings. But there were nothing similar for the loose element. And the paper ring? I couldn't figure out where to put it, and since it was thorn...I thought it unlikely that it would stay in position. So after feeling around a bit I tried a carefully weighted amount of violence and pressed the fourth element downward into the lens....and with a "click" sound it got stuck in position! Testing it...things moved OK. I replaced all other parts...except for the thorn paper ring. And focusing and zooming worked. Shots was sharp again, and no longer a strange difference between the corners! I've saved a lens with excellent optic (in the mid range around 60-120mm its close to the best primes)...though obviously the build quality could have been better!