Here are more info on these BMC Filters from BMC's website:
BMC air filters are designed and produced to ensure a higher air flow than original paper filters. In F1, for example, BMC cotton filters minimize the loss of air flow pressure passing through the air filter; this way ensures the best conditions for full exploitation of maximum power. Therefore the benefits of replacing the original paper filter with BMC cotton air filter, produced using the same technology and materials as the F1 air filters, are evident.
DESIGN AND MATERIALS
Qualified engineers using advanced software and expert technicians using the latest technologies produce BMC air filters. An F1 filter must be very light, must be made of the best raw materials and must improve performance. For this reason we use only alloy mesh with epoxy coating to ensure protection from petrol fumes and from oxidization due to the humidity of the air. The filtering material is composed of a special cotton gauze soaked with low-viscosity oil to give you the best air permeability.
BMC technical staff has developed a particular production system based on soft rubber moulding
which produces the familiar BMC red filters. They are made in one single piece with no welded joints
in the corners, thus avoiding breaking risks. This system, called "Full Moulding" comes from R&D in F1 and it is significant of BMC air filters' technical and quality specifics.
Here are some pictures. Enjoy!