I think it probably is. Over sock in design, probably nothing underneath.
One of the first performance filters to come on the market was the pancake type. Remove the whole air intake system from a downdraught carb, and attach one of these on the top. Apart from anything else it gave lots more room under the bonnet. Not good in the winter though - too much cold air and the carb can freeze up (not literally, but enough to foul the plugs!).
Circular in design with wire mesh on the outside and inside, and foam in between. Never bothered oiling the one I had (in the days of the Mk2 Escorts). Made a great noise though if nothing else!