I believe it depends on the contract with the company who had them slabbed. Some promotional slabs had no need for a cert number (especially if there is no grade).
Except it doesn't have a guaranteed grade. "Gem Proof" on a promotional slab with one of these in it is just a step up from "Ooh! Shiny!" These were likely sent in quantity for promotional holders by the manufacturer. NGC just supplied the holdering technology and were told what to put on the label. The coins were never graded, just packaged.
They were told what to put on the label? Can I tell them to put 70 on my Morgan's holder? The way I see it, this slab guarantees that 1) the medal is genuine and not a fake you can buy for $1.83 2) the medal contains 1.5 oz silver 3) the medal is in proof condition 4) the medal is 65 or better Someone asked what GEM proof meant on NGC forums and this is the answer: "Thanks for a question. Gem Proof is a term that is only used for certain bulk submissions if the submitter prefers that over a numeric grade. It means that the coins would be numerically graded PF 65 or higher." So no, I don't see just as packaging. But that's not the point. Even if promotional gimmick, why no serial? Does adding a serial increase their cost? Shouldn't they be consistent?
If you have a serial you can put the coin into a registry, either custom or competitive. Also you can look up the coin on the NGC verification page.
wcoins - what they are telling you is correct, those slabs are meaningless. Most of the time slabs like that are used on items being sold on the TV shows to people who really don't have a clue about what they are buying - and paying several times more than they are actually worth. As for the no serial number issue - I have no idea as to why. But it's not unusual nor am I surprised by it. These things, these slabs, are mass marketing at its best.
No serial number because the company that ordered the promotional slabs (i.e., packaging) didn't want it, and they specified exactly what was to be on the holder. No grading took place, and NGC agreed to put the text on the holder of all such pieces sent in the order. For the "Gem Proof" piece with the serial number, this was some other private issue round that was sent for bulk grading and a promotional label, which costs more. You will occasionally see actual coins in "Brilliant Uncirculated" holders, as well, typically associated with a hoard of some sort. The point isn't to grade them as much as to add value with packaging and show that they're part of a hoard and are authentic, uncirculated pieces.