Hi RJ, and welcome to the forum.
You've got an interesting Luger. Thanks for posting pictures.
Tom, the frame s/n is 6667
o and the rear toggle is "67". There is the "GERMANY" stamp on the front of frame.
I also can't see a Crown/N proof mark. It is likely that these were shipped incomplete to the USA; the long barrel installed here and the tangent sight recycled from an ARTY?
The "O" suffix block does correspond to manufacture of DWM Alphabet Commercial Lugers in 1925.
These Lugers with modified barrels were advertised extensively in the Weimar era. Here's a reference from the Chicago based Peter Von Frantzius catalogs of 1923-4:
http://www.landofborchardt.com/1923fvf_catalog.html
Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.