I have never attempted to take a FWB apart. I have 2 150's and luckily both work fine but even Paul Watts was looking for parts of fix a 150 and it is hard to imagine a better airgun smith. I have seen 300S trigger asse4mbies come up for sale, albeit rarely but never a 150. They could be interchangeable, I just do not know.
Frank, Jim , Tom and others will know more than I about them.. Good luck.