To be honest I have no idea and I am not taking it apart to find out, its not easy to take a photo of a curved peice of glass as you can see by the reducer sitting behind it.
All I can say is its not convex on both side you can see that in the reflection and the back is not 3 x longer than the front having a close look at it when it was out the back is about - .5 to 1 x concave from the front the type of len it is cemented or not I dont know.
I did compare it with the 6.3 reducer on a lamp nothing to scientific and it acted the same way if anything it may have been a little sharper.
But to say its a simple doublet IMHO would be a bit of a stretch.
I have had the unit for about 8 weeks and it has transformed my c11 it is far superior to my 6.3 yes as you have stated it does get a bit of coma if pushed to f4 but its easily cropped out but non at f7 I use min arou d the f5 to 6 ish
They make your scope very flexible.
The only down side I see with one of these is your collimation must be very good as at fast speeds the coma gets a lot worse if its out and its not to bad if its good
They only work on the 3 inche back of the c11 c14 and equivalent meade but if you have one of these scopes its a great bit of kit and gives great wide veiws no false colour that I can see.
I would also add this the only reason that I would think that Lumicon would stop making these is that it is a bit of a limited market the 6.3 work very well in all other scopes as well as the c11 and c14 they just dont work as well in the c11 as this one does also the 6.3 is cheaper.