Just for information for users of several other binocular brands other than Vortex, this doubler design will fit on any 40-43 mm eyecup. I bought one the other day for my Leica Geovid 10X42 for a bit of a boost when not packing the spotting scope. Very impressed so far.

Very interesting...

my buddy had one for his Swaro's but it was a pain in the butt because you had to unscrew the eye piece each time you used it 

Wonder if it would fit on my Bausch and Lomb's

This unit is a friction fit twist on/off so it is very quick. Build quality seems first rate