I think its called the "Greener" cross bolt which was used by several manufacturers. My Baikal IZH54 SxS has one.

Now I come to look more closely I see that its a concealed bolt as you explained and in that case the Greener type is probably incorrect.

Last edited by Thruxton; 07/22/21 06:15 AM. Reason: Additional information.
As our language becomes impoverished,,our thinking shrinks to fit.