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My uncle's ID Tag shows him as a member of 4. Ersatz-Kompanie SS-Standarte "Deutschland" SS-Verfuegungstruppe. His eventual assignment was with 6. Kompanie "Der Fuehrer". I'm thinking the Ersatz Units were like the Replacement Training Units in the US Military. Can anyone provide me with some information on the Ersatz Units, or steer me towards a good reference? I do read German.
Any information on the Ersatz Unit for SS-Standarte Deutschland would also be appreciated.

Ersatz means literally replacement, so your assumption is correct. However Ersatz was used also for units that were a mix or combination of forces. But in your case the first version is correct