Zenit with 3 screws

I just got bit-up Zenit TTL other wise regular except it has 3 screws instead of 4 screws to hold M42 ring. I never see this modification. Every Zenit, Zorki, or FED what I know have 4 screws. Any ideas what is "wrong" with this camera?

http://www.ussrphoto.com/UserContent/12122011_DSCF0879.JPG

http://www.ussrphoto.com/UserContent/12122011_DSCF0879.JPG

Hi Okynek,

Three screws are quite normal for early Zenit-TTLs. I think they changed to four screws around 1979/80.

David.

These 3 screw series correspond to the first series of TTL cameras Because photoelectric cell is positioned just above the space os the two top screws. In the next series the assemby was changed again to four screws. because the photocell was redesigned and the cameras with three screws often became with its mount loose.
Regards LP

Thank you David. I did not know this.
But now reading Zenit TTL pages on Aidas Web side I start understand his notes about 3 screws .

http://www.sovietcams.com/index.php?1439868397

Thank you Luiz! Wealth of information!!!
I was thinking that KMZ was trying to cut costs of production [:)]
Now I know!

Please do not forget there were also TTL vessions from BelOMO.
The very first ones (proto series had rotating speed dial from EM
LP