There are different versions of the FWB150 foresight. Some are secured using a Phillips headed bolt, whilst the one on mine uses a pin with wedge machined on the underside, this slides into a groove machined near the muzzle and retains the foresight assembly.

I think the pin model is earlier. Without the groove it wouldn't be possible to use this version on a plain barrel? Not having seen the screw version I'm not sure if they are interchangeable, but something to consider to stop you buying an unsuitable part.