The photometric evolution of M31-RV has been investigated on 1447 plates of the Andromeda galaxy obtained over half a century with the Asiago telescopes. M31-RV is a gigantic stellar explosion that occurred during 1988 in the Bulge of M31 and that was characterized by the appearance for a few months of an M supergiant reaching M_bol_=-10. The 1988 outburst has been positively detected on Asiago plates, and it has been the only such event recorded over the period covered by the plates (1942-1993). In particular, an alleged previous outburst in 1967 is excluded by the more numerous and deeper Asiago plates, with relevant implication for the interpretative models of this unique event.