I've had very good experiences with Krebs. I've seen some Krebs rifles - even higher end models, and they didn't have a "Krebs" stamp on them. I don't know that they are supposed to.
As for the certificate - that can be documentation that the rifle was done by Krebs, if that's what you're looking for.
They may have had trouble getting the Saiga for the AK-103. EAA stopped importing them, so they weren't coming into the country. I don't know the situation regarding the 6-month wait, but they may have been hoping for some to arrive.