I might be able to assist, but do you have a left side up/down type safety lever? An "over-the-top" safety lever type like on a Mauser 98 - or a right side horizontal swing (like a Winchester Model 70)? I have seen all three on pictures on Internet - I am not sure whether those or yours were all factory original - might be after market additions? I have never owned a BRNO Model 21, although I have owned several Mauser 98 types, and I have a BSA "Hunter" here, if they are similar.
It would be typical that the safety lever would engage on the front edge of that cocking piece - inside the bolt shroud, along the upper portion of the cocking piece - the firing pin screws into the cocking piece - the trigger sear also hold back that cocking piece by grabbing on that lower surface - at least for Mauser 98, you want the safety lever to slightly pull that cocking piece off the trigger sear, when set correctly. Is also typical that the safety, when "ON", also locks the bolt handle "shut" - often by locking into a notch on the rear of the bolt body. So, in almost all cases, you would have to unscrew the bolt shroud from the bolt body - with the cocking piece and firing pin inside that. Then remove the bolt shroud from the cocking piece, often having to compress the main spring to do so - usually at some point there, the safety lever gets loose or falls out.