The Manufacturer ID is a predetermined value programmed into the decoder that the point of manufacture that identifies who made the decoder. It only identifies a manufacturer, not the type of decoder. The Manufacturer ID is assigned by the NMRA.
NMRA Manufacturer ID Numbers
As part of the NMRA DCC specifications, each manufacturer is assigned a number, or Manufacturer ID. (The same way your car's VIN has codes to identify the manufacturer/country, which are assigned by the Society of Automotive Engineers.) This identifies the decoder's manufacturer.
That number is burned into the programming of the decoder during manufacture, and is used to identify who made a decoder. Often, DCC equipped locomotives don't identify the OEM decoder supplier in their instructions, so this is a way to determine what was installed. Additional steps may be required to further identify the type and model of the decoder, using the manufacturer's instructions .
The complete list can be found at the NMRA site, DCC Manufacturers ID Numbers. The NMRA list would be the most up to date reference on manufacturer ID numbers.
NMRA Manufacturer ID: 18
Unknown Decoder under JMRI
If a decoder reports as a Massooth Elektronik GmbH (and it is not), the answer is that JMRI's Preferences/Defaults has the "Programmer" set to "Internal" or a Simulator connection (in Preferences->Connections).
Internal or Simulator connections always return 123 for every CV read.
The JMRI team has put together a comprehensive listing of Decoder Identifications by Manufacturer or OEM
NMRA Manufacturer ID list S-9.2.2 Appendix A: Manufacturer ID codes as assigned by the NMRA
Soundtraxx Decoder Identification
If the decoder is a known Soundtraxx product, see the SoundTraxx Product Identification guide.
This guide covers the older Tsunami products, newer ones may be more difficult. For more information read the Soundtraxx article.