This is a 1967 BSA Hornet running twin 1.50" OD header high pipes into 1.50" inlet x 18" long "Quiet Core" cone/reverse cone megaphone mufflers. Part number is MC-1500-18Q-SS. Compare this video to the one with the same bike running our 12" straight core mufflers.
The Quiet Cores have a unique chambered design that effectively redirects the exhaust to help cancel sound still retain acceptable performance. The outlet also features and reduced diameter to further decrease sound levels. These two features working together are much more effective than either individually.