On the U.S. Army Map Service (AMS C762) 1:50,000 maps made of Iceland 1948–1951, there is a scale marking at the top which shows degrees, I suspect angles. In the image below it shows 25°–29°. It increases to the left, which suggests it might refer to degrees longitude west, but the numbers are not correct; the map covers 18°W–18°30'W. It's not a magnetic declination either, which is shown elsewhere on the collar in the usual way. Other maps in the series have a similar scale in the same place, but the numbers may be different. What does this angle scale mean?
I believe it is actually used to generate a Magnetic north line, using the reference point P on the bottom edge of the map. At least that's how I interpreted the statement in the declination legend (shown below) which seems to refer to the scale you mentioned.
I picked a sheet a random. At the time the map was done the angle was 31.25 (+ 2.5 degrees?) degrees from grid north, very close to the middle of the equivalent scale I saw in that sheet.
Presumably they added a few degrees either side to extend the usability of the map into future years.