1944 Italian Socialist Republic Sassone #470 Type I Overprint???? Any Thoughts?

I need help on this stamp. My research indicates that it is the 1944 Italian Socialist Republic - Sassone catalogue #470 type I overprint - Brescia printing. Robert A. Seigel Auctions sold one of these for $700. Catalogue value is $5,700.

https://stampauctionnetwork.com/y/y92834.cfm

Stamp is not listed in Scotts.

In September 1943 Italy surrendered, but Mussolini was rescued from imprisonment by the Germans and headed what became known as the Italian Social Republic in the northern half of Italy. At first existing stocks of Italian stamps were used as postal services recovered, but in December 1943 elements of the Guarda Nazionale Repubblicana stationed in Brescia caused existing values of the definitive and air mail stamps to be overprinted G.N.R.. 5 days after work started orders came to cease overprinting.

Anyone have any thoughts about this item.

Hi.
$700 was for a set of 16 stamps,not for a single stamp.
Yours doesn't seem to be a key value.
This one ( Sassone #483/I) is a key value and was sold on Delcampe for 45€