Do you mean the rectangular holes that guide the punches, or that receive the punch below the card (the die)? Or are there square rods that carry the buttons, in a square hole? The obvious solution would be a solvent. Paint thinner might work. If not, you may want something stronger, but if so you'd need to remove any plastic parts (like the keycaps). Laquer thinner is potent, I use it a lot, but only on insoluble items. Rectangular holes can be cast, or machined with a broach. The same goes for square holes, but in addition it's possible to drill square holes, using triangular drills. Watt Brothers Tool Works has made these for ages (see https://www.scribd.com/doc/67999953/Watts-Bros-Manual). paul