Mystery SIG Rifle No Mystery After All. Hoax? Scam?

A self-described “noob” (with only one forum post to his name) posted these photos on AR-15.com claiming to have a prototype SIG rifle sold to his friend at SHOT Show. schnee wrote …
P5041780I’m new here but have been around guns for a long time. A few months ago I bought a rifle from my buddy who does marketing for a local firearms manufacturer. He bought the rifle from a guy at the SHOT show in January who was clearing gear out of his booth (I think SIG but could have been someone else. I am reaching out to this board to help me identify the gun because I can’t for the life of me find anything about it anywhere. I called SIG and they said they have never heard of it but wanted me to send it in to NH for identification even though I sent them photos of it. They claim they want a closer look but I think it’s weird that they can’t do it with photos. Frankly I am worried that they might keep it if it’s some kind of prototype (see photos for why I think this).
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Here’s what I know about the rifle: It shoots 7.62×39. It came with 6 clear magazines. It shoots amazingly well–super accurate and never jams. It looks to me like a rifle that SIG calls the 556R, but it is grey like the 551-A1. In the photos it is oiled so it looks blacker than in real life. It has a plastic-like thing next to the ejection port. Almost looks like the G-10 handles on one of my Spyderco knives. See photos below. Any help would be greatly appreciated.
Claiming that a company sold a rifle on the SHOT Show floor immediately struck me as suspicious. Accessories as frequently sold on the last day of the show by companies wanting to ditch the display inventory, but I have not heard of companies selling off their guns. I would be surprised if the show organizers even allowed this and in any case display guns have to be deactivated before they are allowed on the show floor. Large companies like SIG are  definitely not selling off their guns on the show floor. SHOT Show is not a gun show.
Anyway, I did some research into this gun. The first thing I was told that SIG had no plans to launch a gun of this description. The serial number pattern is exclusive to SIG’s USA guns. Further digging showed that the serial number is that of a standard 556R which was sold last year.
At best this was a hoax, at worst someone was trying to scam an unsuspecting person into buying a “secret prototype”. The post on AR-15.com may well have been an attempt at establishing the guns provenance by seeding information about it across the internet.