A RARE and desirbale item indeed, BUT check your reference books carefully.
The item in question is I believe an original, but I believe that it is a post-war Belgian para-commando mount for twin Browning M1919A4 .30 cal or FN MAG 7.62mm machineguns, not a WWII SAS one for the Vickers G.O. (aka Vickers K) in .303".
5 SAS was the WWII Belgian SAS Company under Eddie Blondeel in WWII and this proud tradition carried on post-war in Belgum. Such mounts were usually seen on the Minerva (a clone of Land Rover) which was modified like WWII SAS MB/GPW jeeps for Belgian para-commando use.
The original WWII SAS mounts are NOT obtainable now for love or money. The only originals that I know of are ironically in Brussells, Belgium as the Army Museum there has a COMPLETE original WWII SAS jeep complete with battle damage from German 20mm shells - though in the latest photograph I saw of it, this jeep was minus its special gear - believed to be in storage). The mount for sale is the next best thing to the WWII one, or one can scratch build one using old photos if one wants to be really fussy.
Subtle differences exist between the late WWII and post-war model - but only a fanatic would know the differences. The mid-war 1942 desert SAS twin gun mounts are quite different and do not have the armour or bullet proof windshield (ex-aircraft) for example.
If you buy this amount, your friends will envy you, but just be aware that it is close, but is probably not actually WWII - AS FAR AS I KNOW. I could be wrong, but I have done a LOT of research on the SAS jeeps and I think I am right on this.
The above is not to be negative about the seller. It is understandable that MOST people, including the seller, would believe this to be the WWII SAS version.