Note: Don't trust this information. It doesn't make sense. The front end FET is probably a dual FET and the 2N3822 is a single FET.
Tektronix part number 151-1011-01 is a 2N3822. It is used as the front-end high-impedance buffer in the latest versions of the 321A. The 151-1011-01 is also used in the 7B50 in serial numbers B010100 to B039999, and in the 7B51 in serial numbers B010100 to B039999.