What is a K&N Airforce Air Filter Wrap?
This is indeed a pre-filter, to be used as a filter wrap.  It is often used in off road situations where the environment is very bad.  These wraps are, contrary to the nylon dry chargers, not water repellent.  However, these foam wraps typically have better filter capabilities compared to the Drychargers.  The red foam is supposed to be oiled, the grey foam is not.

Red colored pre-cleaner filter wraps are made from large, open cell foam that is oiled. They will stop 90% of the dirt, yet only add a 2% restriction to the airflow of the filter. Our red colored Airforce Pre-Cleaners are designed to be cleaned with K&N Cleaner or soap and water then re-oiled with K&N air filter oil (99-0504).

These wraps are NOT to be used as a standalone filter.

Product Specifications
Closed Top    Yes
Color    Red
Height    6 in (152 mm)
Inside Diameter    3.75 in (95 mm)
Material Used    Foam
Package Quantity    1 x Foam filter

Product Box Height    4 in (102 mm)
Product Box Length    0.15 in (4 mm)
Product Box Width    8.5 in (216 mm)
Wrap Type    Airforce Pre-Cleaner

This air filter pre-filter is meant to fit the following K&N Filters:
 rb-0520, rb-0610, rb-0710, rb-0720, rd-0460, ru-0510, ru-0520, ru-0610, ru-0620, ru-0810, ru-1040, ru-1050, ru-1060, ru-1070, ru-1080, ru-1090, ru-1410, ru1420, ru-1470, ru-1620, ru-1720, ru-1780, ru-3030, ru-3190, bd-3303, ha-0510, ru-1750