Many of them were made with double shot injection molding. Rather than painting the label on the keycap, the label was molded into the key as a different color

Many of them were made with double shot injection molding. Rather than painting the label on the keycap, the label was molded into the key as a different color plastic. This is generally in keeping with HP's overall level of engineering that went into these calculators. They used a series of custom microprocessors from the original HP35 on forward, and spent a great deal of time on numerical analysis to make sure the calculators produced good results. William Kahan was involved in this work for years. (Kahan was chief architect of IEEE-754, among his many other contributions to numerical analysis.)