Not necessarily true. It is completely valid to write "None" in your bound book if the gun doesn't have a serial number. In fact I discussed the very topic of guns with no serial numbers with the ATF compliance examiner during my recent annual compliance inspection.
Again, if the serial number was obliterated, gun is contraband. If the gun never had a serial number, not necessarily a problem - gun could be completely legal. It's represented in the OP as a Glock, so if it has a genuine Glock frame, and the pocket where the serial number tag goes is empty, it's almost certainly a contraband gun. The OP said it didn't have a serial number on the frame in the typical spot, but didn't indicate if there was a gouged out hole there, whether or not the frame even had the pocket where the serial number goes, if the serial number was elsewhere on the frame, etc.
It sounds like the gun was also missing the Smyrna, GA import markings, so (if it is a genuine Glock frame and not a 3rd party frame) I suspect the gun perhaps entered the country "unofficially". It is possible the serial number is elsewhere on the frame, or possible that in whatever country/market the gun was made for they don't require serial numbers to be on the frame, but instead want them on the barrel. I know some countries regulate barrels the way we regulate frames.
If it hasn't run afoul of some law making it contraband, this gun might have some interesting collector value. Would be interesting to see some pictures of this thing - but I'm sure it's long gone.