In the revised designation system, as explicated by Still in Weimar Lugers, this is an Alphabet Commercial. It will be crucial to know the calibre of the gun. The 3-3/4" lengths were manufactured in .30 Luger, but Still notes that a small number of the r block guns were manufactured with 4" barrels in 9mm, for military or police use. This would be a fairly uncommon Luger.
Pictures would be very useful here. If you determine that the gun is 9mm, do a search thrugh the forum on "sear safety" and "magazine safety" and see if it has either of those.
According to Still, the DWM Alphabet Commercial r block guns were manufactured in 1928, nearly at the end of true DWM manufacture--BKIW, the company's successor, took over production in the s block.
Congratulations on owning a very interesting Luger.
--Dwight
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