Actually Prather did not get it right, many of the Prather gauges rock, this makes it very difficult to read , when the mount rocks side to side, even a tiny amount, its makes reading and setting pitch near impossible, the fix was to make the prop mount piece wider on both sides so it cover the milled slots on both sides.
This part is way better and way more stable than the stock unit, We made these from aluminum and delrin, both worked perfectly
Randy