These are the stock modems/basebands, packaged as flashable zips.
Supports Model d800 ONLY!DownloadMore on XDA
These are flashable packages that install stock basebands/modems. There is a zip for each stock d800 ROM.
The radio contained in the 20c KK modem image has an issue with SMS messages being delayed. The modded modem patches in the old radio file to correct the bug. However, since the new modem files are kept, sensors in 20c based KK ROMs continue to work.
The only change this makes is to flash the relevant modem partition.
Your ROM must support the baseband you flash. A mismatch will result in certain sensors not working (e.g. GPS) or even worse, you may end up in a bootloop. Only flash the baseband recommended by your ROM developer.
You can't just use the newest version to "fix" things.
ROMs that require the 20c KK Modem (for rotation to work) cause SMS messages to randomly get stuck, causing ~2 minute delay when sending. The modded modem resolves this issue while keeping sensors fully functional.