Boy, that's an oddity...... My gut instinct is that it is a genuine plate as well. I don't think it would be absolutely unheard of for a few to slip out of the IR factory without the date marking. I do think however not having an IR date code will limit the collectibility of the piece, as a correct IR date code is what makes a buyer/owner more comfortable owning such a plate. If it's for your collection to keep, I would feel confident it is authentic but it might be a tougher sell in the future. With all the reproduction Mobil pp's in particular, it's hard not to be overly cautious when buying one anymore.