Heat treated steel bushings. The sizing diameters are hand polished with a surface hardness of Rc 60-62 to reduce sizing effort. Bushings are available throughout the range of .185" thru .368" in .001" increments. The last three digits of the part (#RD73XXX) denotes bushing size.
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Brownells Christmas , Issue:2X, Page:014
Sinclair, Issue:1B, Page:043