This is a Cokin A size Linear Polarizer, it fits all Cokin A holders, sliding into the furthest back slot to fit flush to the holder. It can be rotated using its serrated edge. This Linear polarizer is used to saturate colours, darken blue skies & reduce reflections.
eBay
Phone Clip Circular Polarizer for iPhone Samsung Universal Portable Polarizer Camera Lens 37MM
Aliexpress UK
A polarizing filter arranges, and filters, this directionally polarized light perpendicularly to the reflected light, allowing for the absorption of much of this light. A circular polarizer only differs from a linear polarizer in that it contains a quarter wave plane in addition to the linear polarizing filter.
eBay
Phone Clip Circular Polarizer for iPhone Samsung Universal Portable Polarizer Camera Lens 52MM
Aliexpress UK
eBay
K&F Concept MC CPL Circular Polarizer Filter Ultra Slim Multi Coated
Aliexpress UK
Note this is for use on Cokins professional system holder only, it is not a screw in filter. This filter is in very good condition, there are no scratches or marks on the glass.
eBay
SmallRig 52mm Magnetic Circular Polarizers Filter Kit, HD Optical Glass Circular Magnetic CPL
Aliexpress UK
Complete kit including 49mm and 67mm lens adapters. Uk only sale.
eBay
Freewell Circular Polarizer CPL Camera Lens Filter Compatible with Mini 4 Pro
Aliexpress UK
As above, great filter.
eBay
CPL Filter Ultra Slim Optics Multi Coated Circular Polarizer Camera Lens Filter 37mm 49mm 52mm 58mm
Aliexpress UK
Cokin P160 Linear Polariser Filter with case vgc
eBay
K&F CONCEPT CPL Camera Lens Filter Ultra Slim Optics Multi Coated Circular Polarizer 37mm 39mm 49mm
Aliexpress UK
A160 Polarizer. Classic Cokin filter. For the Cokin filter system. I can spot a couple of very very small and light surface blemishes, but they are hardly noticeable and don't effect the performance.
eBay
NEEWER 2 in 1 Variable ND Filter ND2–ND32&CPL Filter Circular Polarizer Filter No X Cross/30 Layer
Aliexpress UK