Such a slab can come about for three reason. One they tended to us such a label on the give-a-way slabs they use to hand out to their paid collectors club members, but that probably isn't the case because I think they all mention the collectors club and you didn't mention that.
Two, slabs that were submitted as part of a big promotion by the TV coin shows would often be slabbed without a numeric grade as long as they were uncirculated.
Three, dealers who make large bulk submissions have the option of specifying a minimum grade. Any piece not reaching that grade would be slabbed in one of these non-numeric grade holders. The percentage of coins that reached the specified minimum grade would deternine how much they paid per coin for the grading. If the percentage achieved was high enough they could even get the non-numerics back at no charge.