What ist this component on early network cards with a slit in its package?
It has a designator of C38 which means it is a high voltage capacitor with a spark gap.
Typically the coaxial cable interface is isolated from the PC chassis ground, with the exception that the cable shield is connected to PC chassis via a 1 Mohm resistor in parallel with a 10nF high voltagre capacitor. The capacitor voltage rating typically is 1-2 kilovolts. The 10nF capacitor slows down the AC transients and the 1Mohm slowly discharges the charges that have accumulated. In the event of high surge voltage, it is discharged to PC chassis via the spark gap.