Hello Alain
For the Krystall I can confirm the existence of this variant. Alexander Schulz showed one in his book "Zenit" from 2003 with the s/n 61021914. When I talked to him about this item he said that he had only found this one and never seen an other later.
Also Alexandr on www.fotoua.com shows one of them at this link:
http://fotoua.com/1cameratip.php?seek2=143&seek1=140&usl=&usl1=&seek3=2&rd=4&st=7He writes that the s/n observed goes from 61017708 to 62032739. The rarity code "R4" shows that's a uncommon camera.
Aidas shows one of them with s/n 62009728 at:
http://www.sovietcams.com/index.php?1040184497(well, maybe it's not exactly the same type of camera or just a variation of it?)
In the book "1200 cameras" you can find one of this kind on page 136 (or has it a chroming like the earlier Kristall 2 prototypes from 1960?). I can't read the text ... :-/
After all it's a very nice camera, one of the ones I like the most in the Zenit collection.
Best wishes - Guido