  The 2015 LHCb discovery of a structure (denoted by $P_c^+$) decaying in
$J/\psi \,p$ and conjectured to be a penta-quark state, has triggered a renewed
interest in the question of possible existence of multi-quark states not
predicted by the naive quark model. In this talk we present some considerations
on $P_c$ photo-production experiments, aimed at testing its multi-quark
interpretation in the framework of a 40-years-old string-junction picture that
allows a unified description of baryons, tetra-, and penta-quark states.
