The serial numbers listed in the robots are all listed as drives - so your problem is not an issue with tape drive serialization.
.
When I search the NetBackup mappings file on my test server:
J:\>findstr /i "[version" "D:\Program Files\Veritas\NetBackup\var\global\device_mappings.txt"
[VERSION:1.124]
J:\>findstr /i "changer.*quantum" "D:\Program Files\Veritas\NetBackup\var\global\device_mappings.txt"
[CHANGER:Quantum DLT4500]
[CHANGER:Quantum DLT4700]
[CHANGER:Quantum DLTStor 4108]
[CHANGER:Quantum DLTStor 4314]
[CHANGER:Quantum DLTStor 7108]
[CHANGER:Quantum DLTStor 7314]
[CHANGER:Quantum DX30]
[CHANGER:Quantum DX3000]
[CHANGER:Quantum DX5000]
[CHANGER:Quantum DXi Series]
[CHANGER:Quantum DX100]
[CHANGER:Quantum L500]
[CHANGER:Quantum Powerstor L200]
[CHANGER:Quantum DLTStor 108]
[CHANGER:Quantum DLT Changer]
[CHANGER:Quantum Powerstor L500]
[CHANGER:Quantum L200]
[CHANGER:Quantum PX500]
[CHANGER:Quantum PX720]
[CHANGER:Quantum Scalar 40]
[CHANGER:Quantum Scalar50]
[CHANGER:Quantum Scalar i2000]
[CHANGER:Quantum Scalar i6000]
[CHANGER:Quantum Scalar i500]
[CHANGER:Quantum Superloader]
[CHANGER:Quantum ValueLoader]
[CHANGER:Quantum ValueLoader]
[CHANGER:Quantum ValueLoader]
[CHANGER:Quantum]
...and then search your scan output:
J:\>find /i "inquiry" scan.txt
---------- SCAN.TXT
Inquiry : "IBM ULT3580-HH5 E6Q3"
Inquiry : "QUANTUM DXi6700 100A"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "IBM ULT3580-HH5 E6Q3"
Inquiry : "IBM 3573-TL C.50"
Inquiry : "IBM ULT3580-HH5 E6Q3"
Inquiry : "QUANTUM DXi6700 100A"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "HP Ultrium 5-SCSI I30Z"
Inquiry : "IBM ULT3580-HH5 E6Q3"
Inquiry : "IBM 3573-TL C.50"
Inquiry : "IBM ULT3580-HH5 E6Q3"
Inquiry : "IBM ULT3580-HH5 E6Q3"
.
Maybe you need an updated mappings files, as per:
https://support.symantec.com/en_US/article.TECH162165.html
.
The HCL for Quantum:
http://origin-symwisedownload.symantec.com/resources/sites/SYMWISE/content/live/SOLUTIONS/76000/TECH76495/en_US/nbu_76_hcl.html#virtual_tape_libraries_drives-virtual_tape_libraries_drives_-_vendor_compatibility-quantum
...shows that the inquiry string should be "QUANTUM^DXi", and your "scan" output shows a inquiry response of "QUANTUM DXi6700 100A", which looks like it should partial match the beginning of the string - assuming "^" means a space.
.
What is your version of mappings file?
findstr /i "[version" "D:\Program Files\Veritas\NetBackup\var\global\device_mappings.txt"