So first of, thanks go out to Mijzelf for all of his help and direction on this. I was able to install the latest NSA-220 Plus firmware (3.25AFG) onto my non Plus model NSA-220. To do this I used the USB key firmware updgrade file 312AFB0C0_for_2.30_or_above.exe ftp://ftp.zyxel-tech.de/2.new_mirror/NSA-220/firmware/NSA-220_3.12(AFB.0)C0.zip
. I uncompressed that file onto a USB key. I then downloaded the latest NSA-220 Plus firmware 325AFG0C0.bin ftp://ftp.zyxel-tech.de/2.new_mirror/NSA-220_Plus/firmware/NSA-220%20Plus_3.25(AFG.0)C0.zip
and renamed it to ras.bin and replaced the ras.bin on the USB key with this one. I then modified 2 files on the USB key. The first one 'version' I opened in a text editor and changed the text from v3.12(AFB.0) to v.3.25(AFG.0). I then used the text editor to change 'Upgrade_FW' from 1b44c6d1bd301ea79ac956e2f16e3757 to 672cf6822b5bfc7834f5230054a115df .
So with the new ras.bin, version, and Upgrade_FW files copied onto the USB key in the nsa220_fw directory, I powered down my NSA-220, inserted the usb key in the front left usb slot, and then powered the unit back on. After about 5 minutes it finished and powered itself down. I removed the USB key, and powered the NSA220 back up, and I now have a non Plus model NSA-220 running firmware 3.25AFG.