Thanks for your email.
LEE Filters do not make a reverse ND grad. We do get asked for them a lot, but there are two reasons why we do not offer them.
Firstly, we think that reverse grads, as they are sold by some of our competitors, are not really a lot of use. All they really do is put a hard grad on the horizon line. The colour fades again at the top of the filter but it would have to be a very wide lens indeed to get both parts of the filter in shot.
What people really need for those 'sun below the horizon' shots is an ND stripe, and this can be achieved using two existing ND grads.
Take a look at this link http://www.onlandscape.co.uk/2010/12/ad-hoc-reverse-nd-grad-filters/ which was not created by us, but explains the technique well.
This technique really does work and is certainly worth a try, especially if you already have the ND grad filters.
The second reason we don't offer reverse grads is because we cannot actually make them using our production techniques, but even if we could, I doubt we would for the reasons given above
I hope this explains,