Glad it was interesting to you...
What are the errors when it boots? Please paste them in a new topic (unless related to no primary or secondary image)
The spare unit was upgraded at some point where the prod unit appears to have its original factory firmware from when it shipped new in box. For the actual differences, you will need to read the release documents. Both builds are extremely outdated. Neither of these units are running the latest build... Even the latest build is still end of life and not supported.
You can download the latest build for that end of life product and install it. Not certain these are the right files for that platform, but these are what I used to use on the old FastIron WS units (before we replaced them all with newer units).
copy tftp flash 10.1.2.3 fgz05000.bin bootrom
copy tftp flash 10.1.2.3 fws07400p.bin primary
You would be a LOT better off with the ICX 7150 or 7250 series units, which are still supported.
