Comments about Leica Leica Lens Hood for the 50mm f/2.8 M Lens, Silver:
Purchased primarily to protect the 50/2 Summicron's soft coating from dirt and scratches. Acceptable protection from flare. Very compact when compared to vented hood - 12585, and expensive older hoods ITDOO and IROOA. Screws on so stays put. Clean, good looking on top of a modern Elmar or older Summicron. Provides compact camera and lens combination.
Only real problem is vignetting at wide open apertures. May be less pronounced when lens is stopped down to f8.0 and smaller.
A single old filter ring, or double rings, without the glass, may give you the same protection.
Did not fully meet expectations, but usable.