I had this 1960s Czech bass in last week because the neck had popped out. Amongst other things, this was possibly due to the purfling around the neck joint being fitted so deep that there was no proper support for the button, which had come off along with the neck. We decided that given the severity of the break vs the overall value of the instrument, but far the safest and most sensible thing to do was to bolt the neck back in. Hopefully it should remain solid for many decades to come.