I have just purchased what I thought was a used Hasselblad Acute Matte 42165 focusing screen. There are no identifying marks on it and it looks identical to the screen in my 500 El/M. Is there a way to tell if this is an Acute Matte screen? I can see no difference in the brightness of either screen and I am wondering if this a regular screen in an Acute Matte box. Any advice would be greatly appreciated.