Lumicon 2 Inch Neutral Density 13 Filter
I bought two of these filters. Mechanically and optically they are both excellent quality. I did find that retaining rings loosened during shipping and the filters rattled a bit. The ring was easy to tighten and both are working well.
My only criticism would be that by my measurements I have to increase exposure times by 4.8x instead of the expected 7.7x to match the unfiltered image intensity. This yields a transmission ratio of 21% instead of the stated 13%. If photometrics are important to you then you should be aware of this. Still, they are excellent quality 21% filters and if that will suit you then you will be happy with this filter.
Order Dec 27th, now Jan 24th and still hasn't arrived. My advice is order elsewhere!