CB,
According to the INSTALLATION INSTRUCTIONS sheet for the 970 series electronic tachometers with 990 series ignition monitors, the tachometer itself is listed as Model 977 Series (which feature a unique pedestal head assembly). There is no letter designation for the tachometer.
Are you referring to the ignition monitor ? If so, use Model 990-B for Standard Battery Ignition installations.
Kent
'68 Malibu ZZ502/502
