I am seeing a lot of normal production Jupiter lenses on Ebay being advertised as ZK Sonnars and BK Biogons. Someone in Russia must be making Name Rings with ZK and BK inscribed, with serial numbers and markings to fool potential buyers. These lenses are going at a premium. Sellers used to put phony Carl Zeiss Jena name rings on Jupiters and try to sell them as Sonnars. Showing them as ZK and BK lenses- probably easier, as there is not as much documentation on differentiating them from authentic lenses.
We'll start with my 1952 Jupiter-12, which is a Carl Zeiss Jena Biogon from 1943.
Early coated CZJ 3.5cm F2.8 Biogon on the Left, my $50 J-12 on the right.

It's in the book- the Serial Number is stamped on the rear element and is visible with the rear lens cap off.

Also note the thicker focus ring on the early J-12, the later lenses -that I have seen marked BK- have the thinner ring of the later J-12.
If a lens is being advertised as a BK Biogon and does not have the Serial Number on the rear group being displayed, DO NOT buy it- it is a later Jupiter-12. IF it is a Jupiter-12, and HAS the SN like mine- Grab it. It has Zeiss optics. This 1952 J-12 is as good as the Coated Biogon shown alongside of it.
We'll start with my 1952 Jupiter-12, which is a Carl Zeiss Jena Biogon from 1943.
Early coated CZJ 3.5cm F2.8 Biogon on the Left, my $50 J-12 on the right.

It's in the book- the Serial Number is stamped on the rear element and is visible with the rear lens cap off.

Also note the thicker focus ring on the early J-12, the later lenses -that I have seen marked BK- have the thinner ring of the later J-12.
If a lens is being advertised as a BK Biogon and does not have the Serial Number on the rear group being displayed, DO NOT buy it- it is a later Jupiter-12. IF it is a Jupiter-12, and HAS the SN like mine- Grab it. It has Zeiss optics. This 1952 J-12 is as good as the Coated Biogon shown alongside of it.