Their drawing should state what standards they use in creating their drawings (yeah, I know, lots of companies, even big one, have lousy drawing formats that leave you guessing). Rz is a surface roughness parameter that has about 4 different definitions depending on which standard you use. You can not with any degree of accuracy convert Rz to an Rmax or an Ra as each roughness parameter is derived from the exact profile being measured. A sine wave profile with the same Ra as a triangle wave profile will have very different Rmax and Rz values.
You could take a look at ISO 1302, it has a conversion between current Ra values and obsolete Roughness Grade Numbers. The obsolete number system always starts with the letter "N". An N4 roughness grade number corresponds to 0.2 Ra (in microns). Still no mention of the /Pt12.
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