Old photo (1930) of my new motorcycle with blackburne engine

[Moy46]

Here is a picture of my vintage Magnat Debon, (on the left) with the owner over. Right with his brother FN 500 M67 OHV.
So I bought this bike last month, it is fitted and equipped with an engine blackbune 500ohv twin exhaust and I seek all the information possible on this engine.