I'd ask Lex, Ian. They were used on Nortons from the mid-1930s onwards. The War Office stipulated 'Trials Type, adjustable' handlebars for the 16H so they don't have them but the SWD Big 4 outfits do (presumably because that 120mm stroke engine is a bit of a shaker !
Jeff Hunter makes them but I found his rubber was too soft on my Speedtwin. So I got some punched out of tough Neoprene by "Portmere Rubber" It's not so important on the B4 as they have a handlebar brace to stop the bar moving when you lean on it. So the soft rubber will work just for damping. Ron