Any of the 3/8" stuff is 'nominal' and may require some fitting. Always file on the sight, or blank, base, not the barrel dovetail. It is a good idea to 'break' the sharp edges' on the sight or blank as they tend to be too sharp in the barrel dovetail corners and throw you off for fitting.
BTW, you can file your own blank out of steel, or even brass, just use a piece long enough to have the vise hold it really well and work on the end sticking out, then cut it off when done. A nice durable finish on steel is done by heating it and cold blue, repeat 2 or 3 times, cool and oil. On brass just cold blue darkens it up sorta like old patina.