SincoTC, bingo!
To be honest, I think it is the BL-502 as the BL-503 was a modification of the original BL-500 (besides, the source of the silhouette also describes it as the BL-502.)
I understand history is as follows:
BL-500 first example with 150hp Lyc.O-320
BL-501 not built (planned with 125hp Lyc.O-290)
BL-502 second example with Lyc.O-320 and the landing lights in the wing l/e, larger spinner and recontoured cowling
BL-503 the BL-500 now with 180hp Lyc. O-360 and I donot know whether the landing lights were added.