What Type of 2M mobile antenna is this?
 

I just bought a trunk-mount 2M antenna from a SK's widow, and am not sure
what type it is, though it seems to work really well. It is a 17" whip on an
NMO base, with a quarter-inch stainless steel "'tube" around the whip
starting about 4" from the top, and extending downwards 6" or so. At the
base of the stainless tube, there's a plastic insert to keep the bottom edge
of the tube from touching the whip. A set-screw on the tube's top edge
allows it to slide up and down the whip for adjustment.

Works great at 2M but I have no 440 rig to test it so I don't know if it's a
dual band antenna or not.

Doesn't look very old, but don't see anything like it on the web's antenna
dealers.

Anybody know what model and what the theory of operation is for this one?

Thanks,

Kevin VE9XYZ