52mm drop in polarizers - interchangeable?

Only difference is the paint color, the W and WII drop-ins are physically compatible with all the supertele lenses (the WII paint matches the MkII lenses and MkIII extenders).

Note that your post title mentions drop in polarizers, but your links are to standard holders for 52mm screw-in filters. If you want a CPL, get the drop-in CPL. The holders you linked are for ND filters (or IR, etc.), if you mount a 52mm CPL in one (assuming it's thin enough), you'll have no way to adjust the polarization without removing it (Canon's drop-in CPL has a knob on the outside that's geared to rotate the inserted CPL).

CPL in new white: http://shop.usa.canon.com/shop/en/catalog/product-accessories/ef-lens-accessories/lens-filters/52mm-drop-in-circular-polarizing-filter-pl-c-52wii
 
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I ordered my Tripod Mount Ring C (for the 70-300L) from UniquePhoto, as they were the only ones who had it in stock when I wanted it. Also my first (and only, so far) order with them, but they delivered.
 
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