I recently upgraded my 47mm S.A. to the XL version and am having a bit of a problem.

The lens came in its original box with an attachment or (flange?) that's supposed to fit over the rear of the lens. The rear lens cap fits onto this attachment.

The problem is that it's awkward and requires quite a bit of pressure to fasten this attachment to the rear of the lens, and even then, it doesn't want to stay attached. It's a little scary trying to fasten this attachment, because with the pressure required, it can slip. I sure don't want to scratch the surface of the rear element; yet, without this attachment, there's no way to cover the rear with the lens cap.

There's an "O"-Ring on this attachment that slips over the rear of the lens through three grooves. Perhaps I need to lubricate this ring? But, I wouldn't know what to use.

Any suggestions on dealing with fastening this rear attachment to the lens? Is there a cap available that can protect the rear of the lens without using this attachment?