Actually it's a representation of the Tourist Trophy (IOM) stemming from Howard Davis's successes in the TT in the twenties. Presumably Philip Vincent decided to keep the Mercury figure on his machines as homage to HRD. After all, the HRD makings were more prominent on post-war Vincents up to 1950 until his American agents advised him the "HRD" was a bit too close to "H-D".