I bought this as a replacement for the original brass cap that came on my Leica Elmarit 90mm M lens (dropped that one in the ocean - Darn!). This one works perfectly, never pops off, and keep my glass protected. Very pleased with the purchase.
Granted, maybe my Lumix DMC-LX3 isn't exactly 39mm. It's possible. But what I know is that this lens caps barely stays on. It's very loose and you can see that it doesn't even hug the lens all the way. The push-in nubs on the side actually don't sit flat to the inside face of the cap so that it CAN'T sit flat fully onto the lens.
The push-in button/nubs themselves don't extend past the outer edge of the lens so that they are hard to push in when you do want to remove the cap. It's a rather awkward design — to have buttons sticking out of the side like they do.
So in the end, the caps falls off once in a while because it's so loosly held on.
But it's hard plastic and the buttons seems like they would last a while, so if it fits your lens well, it would be a fine lens cap for the money. They ARE very inexpensive after all.
I bought this for my Leica Summicron 50mm lens. The cap comes off too easily, and it would drop off when barely grazing my arm while I would have the camera slung over my shoulder. I had to return it for a refund. One of the cheaper caps on [@] works much better.