@holmz , You are referring to what Frank Schroder calls cartridge mounting plates. They are made from one of three materials, a phenolic plastic for very compliant cartridges, Certal (an aluminum alloy) for medium compliance cartridges and brass for low compliance cartridges. Most cartridges will do fine on the Certal plate. Cartridges like the Koetsus and the MC Diamond require the brass plate.
The two outside holes are drilled at 1/2" centers, 2.5mm and the center hole is drilled and tapped for an M3 cap screw. I add a finger lift to all my plates. I find using a finger lift is a much more stable and reliable system of handling the tonearm. $12,000 cartridges are at risk. Frank does not add anything to his arms that might resonate including finger lifts and tonearm rests. I added a locking tonearm rest to my turntables plinth. My finger lifts are dampened with heat shrink tubing. They are made from non magnetic stainless steel wire. Drilling a hole in the side of the plates can be a daunting proposition but with a milling machine and the appropriate vise you should not have a problem. I use an interference fit with added high strength loctite. After the wire is mounted I bend it to shape.
This is what I do. All my cartridges are mounted on an appropriate plate. With the cartridge mounted and VTF set I put the stylus down on a rotating old record I do not mind scratching and measure the distance from the record to the top of the cartridge mounting plate. That distance is recorded on a 3 X 5 file card. I the use a Wally Referance tool to set both VTA and Azimuth using this measurement. There is a VTA scale on the tonearm's post. That number is also recorded. This puts the tonearm exactly coplanar with the record surface. I then set tonearm overhang for Lofgren B using a SmarTractor. I scribe a very fine line on the top surface of the mounting plate using the front surface of the arm as a guide. I can now return to the specs for that particular cartridge instantly with just a VTF gauge. Next is the hard part you need special equipment for. I verify both VTA and azimuth with a horizontal USB microscope similar to the WallyScope but much more stable. The WallyScope works but it is a bit of a PITA to use. Getting the stylus in center field and focused with a scope on a wobbly stage takes patience and a very delicate hand. Since I have neither, I constructed a horizontal microscope using the scope and camera of the WallyScope and the entire stage mechanism of an old medical microscope. It is an odd looking affair but much more stable and easier to use than the WallyScope. I have discussed this with J.R. who modified his stage a bit and it is better but still a far cry from a medical microscope. Fortunately, for these measurements you only need the low power objective which is much easier to deal with. The higher powers are needed to assess stylus wear and Zenith. Measuring angles is easy. Amscope has a program for microscopes. You can snap lines and the computer will calculate the angles for you. My standards are a VTA of 92 degrees +- 0.5 degrees and an azimuth of 90 degrees +- 0.5 degrees. All my current cartridges, the Lyra Atlas SL, MC Diamond and the MSL Signature Platinum are within spec so none of the adjustments needed modification and no shims are required. I do not measure Zenith. I do look at the styluses and take pictures of their initial shape for comparison purposes (easier to see wear), all of these cartridges are in spec by eye. At the price of these cartridges, if they were not in spec I would return them with pictures validating the problem. Actually, I would do that with a cartridge at any price. If a cartridge does not match up to it's published specifications it is by definition defective and should be replaced, no shims required.