The "Western Electric" datasheet is named "5998.pdf". The Western Electric datasheet also says "Western Electric 5998" on the first page. The two tubes are identically constructed.
The 5998 lists 120V plate, 125mA, 17,000uMhos, and an amplification factor of 6.2.
The 421A lists 110V plate, 125mA, 20,000uMhos, and an amplifiaction factor of 6.1.
For lower plate voltage and the same current, grid bias will be lower, and that change in operating point will increase transconductance a little.