I had one of the 4-string ones back in the 1970s, and I'm pretty sure it was just a push fit into the two sockets on the top of the bass. You're talking about the steel bar, about 300mm long with two 'spikes' that point down into holes in the bass, yes? Maybe try a drop of penetrating oil around the spike ?
Be interested to see some pictures. Wish I'd kept mine now.