Can you try this?
fdt2.exe -d wpspk -mi -f WP76xx_Release16.0.1_ATT.spk WP76xx_Release16.1_TMO.spk
Please note that after the upgrade, AT!IMAGE should contain those FWs:
at!image?
TYPE SLOT STATUS LRU FAILURES UNIQUE_ID BUILD_ID
FW 1 GOOD 8 0 0 002.098_003 02.37.00.00_ATT
FW 2 GOOD 9 0 1 001.017_001 02.35.02.00_TMO
FW 3 GOOD 7 0 0 002.095_003 02.37.03.00_GENERIC
Max FW images: 3
Active FW image is at slot 2
TYPE SLOT STATUS LRU FAILURES UNIQUE_ID BUILD_ID
PRI FF GOOD 0 0 0 002.098_003 02.37.00.00_ATT
PRI FF GOOD 0 0 0 002.095_003 02.37.03.00_GENERIC
PRI FF GOOD 0 0 0 001.017_001 02.35.02.00_TMO
Max PRI images: 50
OK