A fellow member on Gemology Online board wished to have this message relayed to you. Evidently he is not a usfg member and can’t post.
“According to MDR’s “the Book of Gem Cuts (vol. 2), the #30 is type metal, usually composed of tin, lead, and antimony.”
I have this volume as well and a old MDR catalog that lists the different laps and in the volume 2 #30 is only listed as “Type” nothing else I see. Strange but true.
But in a catalog it is listed as #30 TYPEMETAL with the following description: aluminum backed, precision machined. An excellent lap for Garnet, Topaz and Zircon using Linde A. A good lap for Tourmaline. Hardness 2.0, shipping wt.2lbs.
Also just for further info the #28 is “TIN/TYPEMETAL” which I assume it is a mix of metals and they state as a hardness of 1.8 which I also assume that would mean more lead content giving a slightly lower hardness than the #30.
But then #26 “TIN/LEAD” they state is 50%-50% and a hardness of 2.0.
It appears they had their own hardness scale for the metals, but they did not disclose all the details of the metals.
Maybe possible the gentleman on the GO board knows something more about the MDR laps in the comment “usually composed of tin, lead, and antimony”.
Anyway hope that helps some.