Filters: What does tiffens "SR" stand for?

The “SR” stands for “Self-Rotating”. In other words, the mount allows you to turn the filter to adjust the polarization angle.

Thanks, Self-Rotating makes sence, but isn’t that just what circular does?

Circular polarizers actually have two elements through which the light passes: first, a linear polarizer, then a quarter-wave plate. Most modern autofocus systems and internal light meters don’t function well with linear-polarized light; the quarter-wave plate takes that linear-polarized light from the first element and converts it to circularly polarized. The final result in the picture is very similar. Circular polarizers are just about making your camera’s electronics function properly.

“Self-rotating” is a designation that can apply to either linear or circular polarizing filters. It’s the style of mount. This distinguishes it from a drop-in mount, where you screw a separate filter holder onto the lens and drop in the filter disks.