Thanks to Skinny Shooter for supplying the EaRL - I found:
Bill - THe unit shown attaches to the rail. They also make a scope mounted unit ( non-pivoting Price: $59.95 (colors extra), pivoting version Price:$69.95 )
Picatinny style Price: $48.95
One thing I find disturbing about the Picatinny slider version is they write: <font color="red">
From unit to unit the angle may vary 1 degree or 2 degrees, don’t be alarmed by this.
Note: it is not important to have the bubble exactly aligned as long as the bubble is placed in the same position and alignment each time. It is a common mistake to assume that the accuracy of the repeatability of the device depends on
the exact vertical or horizontal alignment, but it is very important to make sure the location of the bubble is repeated
each and every time you take a shot.</font>
I line my reticle up with a plumb-bob; I want my anti-cant device perfectly level with the reticle so I can quickly verify vertical. I don't want to think about 3.7 degrees left (or was that right????). Since it's not adjustable, I think I'll go with the scope-ring version. The pivoting is a great feature.
The non-pivoting version looks quit a bit like the $15-shipped
AACD (affordable Anti-cant device)
I'm sure the $50 unit is more robust - but I've been happy with my $15 unit.
The pivoting scope version is a must have.