On older guns there was (is) a three position safety. If you pushed the safety forward the gun would automatically return to safe when the gun was opened. If the safety was pulled back the safety could only be put back on safe manually by pushing the safety forward. This was useful if the gun was being used for targets and the shooter didn’t want to have to constantly take the safety off to fire the shotgun. Perhaps there were concerns about the third position being manual and this was a way to disable it. One possible answer but I think that is it.