The Contarex Biogon 4.5/21mm is a legendary lens because Zeiss wanted to achieve the ultimate quality in 35mm lenses. This lens was so expensive that only 4,000 were built between 1958 and 1962. Image quality is just amazing (compared to modern standards, that is). Build quality of the barrel is among the best ever achieved, turning the aperture and focussing rings is smoother than on any lens I ever owned (that includes my Summicrons). This lens is heavier than modern Zeiss glass, ca. 320 gramm (11 ounces) including the adapter.
My copy has perfect glass, perfect mechanics, but some moderate signs of use. The original Zeiss finder coming with the lens is in similar state, both are not in "mint" museum state, but in good to very good condition.
Adapting Contarex lenses to modern cameras is not simple because most of these lenses do not have an aperture ring (aperture was set from the camera body). The 21mm Biogon is an exception because it has an aperture ring. The rear element almost touches the shutter, on a Contarex you had to lock up the mirror and use an accessory viewfinder. It is probably impossible to use this lens on an M5 body. On my M6 the metering is not accurate because the rear element shades the lightmeter sensor.
The adapter coming with this lens is a purpose-built item, there are only very few on this planet, and I own two of these adapters. It is precise, well-built and in mint condition.
As there is no original rear cap for this lens with the adapter it has a deep no-name Leica rear cap. The front lens is protected with a new german Schneider Kreuznach B+W slim UV filter, MRC coated (the best available) and a Schneider snap-on cap.
Price and shipping:
1200 USD (900 Euros) plus shipping from Germany. Payment via Paypal.