This is a replacement airfilter that threads on the 376 series Amal Monobloc carb as well as the 600 series Amal concentric carb. This seies has a center hole. There is a second one that screws on the 900 series Amal concentric carb. This one is offset as shown in the picture. This filter works well on the street models, Metralla & Mercurio and the scrambles model Sherpa S where the frame comes down behind the carb not allowing much room for the filter.