Vintage Industar 50-2 - 50mm f/3.5 - only 65 grams (2.5 oz)
The Industar 50-2 made by Zenit KMZ is quite a tiny little lens - shorter than a roll of film and the front optic is smaller in diameter than a dime. It is a Zeiss Tessar copy with 4 elements and will be quite sharp in the center. They have quite a reputation and punch above their value and are very convenient being both small and lightweight. Without a shade they can flare in some circumstances which can be a good thing if used to creative advantage.
These are of course all from the 1970's and 1980's and are used vintage lenses however they have all be gone through by a camera technician and will have no issues. They will come with both front and rear caps.
M42 mount but can be adapted to other cameras with inexpensive adapters.