There's an interesting thread on Searle's use of the bamboo pen over at the Illustration Art blog, suggesting Searle first dicovered this tool as a prisoner of the Japanese during WW11. The variety of mark-making in Searle's work indicates that he uses a variety of tools, nibs, fountain pens brushes etc.
Artists often wonder how did Searle achieve those marvellous results- what tool did he use? The answer is probably all of them. In this photo below from the display case at the Cartoon Museum's Searle exhibition we can see the different media he used, including a bamboo pen, a variety of dip pens, the wood stain used as ink early on & the famous Mont Blanc fountain pen.