If I'm not mistaken, that's the location of the "mij" roll-stamp typical of guns with a BTFE.  That said, the BTFE gun I have has the mij roll-stamp in the typical SFE location so.. 
 
I contacted the seller about this gun just about the time it became available..he didn't realize the engraving differences and he didn't have any info indicating the gun was in fact, a true prototype.  Here's the left and, underside of the gun.  If I recall correctly, it's a "P" gun.
		 
		
		
		
			
		
		
		
		
		
		
		
	 |