I’m making a reproduction of a Danish modern side chair. The side rails are connected to the front rail using dowels. But there is also some kind of threaded insert that is used to reinforce the joint.

From what I can see without breaking the original apart, there is a threaded insert in the side rail (see picture) and one in the front rail (which can be seen, but not photographed, since it is within the joint). There is some kind of connector that is attached to both of these.

Any ideas about how this might work? Are there other kinds of hardware that I could consider for reinforcing the joint? This could be an application for some kind of knock-down fastener, but I have no experience with them.

Any suggestions would be helpful.