You could well be right, Mel. There were only 4, I think, examples of the SM 87 built. Prototypes according to some sources but this panel looks too finished. I was also thinking the SM 75 - essentially the land version of the 87 - which was used in Spain which may be the connection Bri's source was making. Have checked both Hungarian and Italian sources without being able to confirm.
It would be nice to know for sure
