There is a known limitation to the Product Description device fact where it returns "n/a" because some models are not documented in the following file, which is placed on the device by Apple and holds the hardware specifications for most macOS devices released by Apple:
"/System/Library/PrivateFrameworks/ServerInformation.framework/Versions/A/Resources/en.lproj/SIMachineAttributes.plist"
The following is a custom fact that can be added to policies and deployed to devices that don't reliably pull the right data:
#!/bin/bash
# be sure to us this as a bash script as zsh will interpret the '?' as a wildcard
curl -s https://support-sp.apple.com/sp/product?cc=$(
system_profiler SPHardwareDataType \
| awk '/Serial/ {print $4}' \
| cut -c 9-
) | sed 's|.*<configCode>\(.*\)</configCode>.*|\1|'
Note: This script uses Apple's service site; as a result, if the site is receiving too much traffic, Apple could prevent the functionality of this workaround by putting the information behind a CAPTCHA checker.