the 25 anniversay edition he has is for the 25th aniversary of the 911. below is a link to one sloan has for sale. the one you reffer to is a 1975 25th aniversary 911 that is i think for 25 yrs of porsche being sold in the US.
1989 Porsche 911 25th Anniversary Edition Black metallic /gray 6001 miles | Sloancars
sloan noted the glove box plaque you are looking for but deos not show it. maybe he can snap a pcture and give you the dimensions and as the vintagecarlover says maybe a trohoy shop can make one. this may be an item you will never find w/o a number already engraved on it.