I should have looked first into the list, the link to which Geert posted. There, on page 2 we find:
Häufig verwendete Abkürzungen:
mRv = mit Rang vom
i.R. = in Ruhe
GM = Generalmajor
1.9.1805 mRv. 12.2.1804 GM, 1806 i.R.
means he became Generalmajor on 1.9.1805 (the patent being antedated to 12.2.1804), and pensioned in 1806.