Hopefully you may be able to help!

I have been given an Original 35 without a trigger unit to fix for a friend having been refused by a gun shop!

I managed to get a trigger unit but when I tried to fit it the pin is blocked from sliding through by a spring in the main body. Any ideas ?

Everything else is in place but may have been tinkered with considering the trigger was missing, nothing on you tube seems to explain this issue. The gun breaks but without the trigger unit does not hold the cocking action.