I've been seeing a trend lately to boost the sale price of equipment by changing out the Serial Numbers, Aperture Rings, even fake screws- to boost the price. Most that I've seen are Ebay auctions from the former Soviet Union countries. I've worked on some of these lenses just to inform the buyer that it is a much more common lens than what was advertised. This one for example: a standard Jupiter-3 focus mount: Lens ZK Sonnar Krasnogorsk 1 5 50mm Leica Fed 1947 Made s N 000180 | eBay And another http://www.ebay.com/itm/Lens-ZK-Son...41001998?pt=Camera_Lenses&hash=item2a2cd8570e The focus ring in the standard J-3 has three set screws, the German version has an extra "stop Screw" that sets the stops for close/infinity. I prefer the Russian J-3 design- but th German version is worth more. I have personally converted a Contax mount ZK optics module to LTM using a Jupiter-3 focus mount, and a 1950 Jupiter-3 Contax mount to Leica mount: and would advertise as such. You would not know if the optics module on this one is genuine until unscrewing it from the mount and finding a Serial Number stamped into the rear module. Another "not quite right": 1951 Made Lens MC Jupiter 8 2 50 s N 5111497 M39 Fed ZORKI Leica | eBay The early J-8 focus mounts were constructed differently, and do not have three screws holding the rear section in. This one does, it was made in 1955 or later.