Part 2 (edited to include correct unit mark information)
Many of the small inspectors' marks can be seen in the following pictures. Not all of them are easily visible, and some of them do not accept whitening well.
A holdopen has been added, as required in May of 1913. A pin has been inserted for it from the outside of the frame, seen here properly unblued, just above and to the back of the trigger pin. The small inspector's stamp certifying its addition is seen just below.
...but the sear bar has not been relieved (a retrofit required in mid-1916), as can be seen just to the right of the sideplate.
The grips are numbered to the gun, and have inspector's stamps.
Much has been made of the inferiority of manufacture of Erfurt Lugers, compared to their DWM contemporaries. Here you can see where the tool used to cut the diamonds of the right toggle-knob end shifted between cuts, double-cutting them and killing their points. The same flaw is noted on the left knob as well, but not as strongly.
This Luger has a couple of very strange markings. On the trigger plate, just above the serial number, a rather cryptic figure is stamped, accompanied by a double-struck (and incompletely struck) upper-case Roman M . A similar figure, along with the poorly struck M , is also stamped on the right receiver above the holdopen pin. The M on the receiver is very difficult to see in the photograph, but up close and personal it is assuredly the same stamp as on the trigger plate, inverted. Both sets of stamps are under the blue.
Has anybody seen the like?
This 191 Errfurt is unit marked, 14.A.F.II.8.H.6.
This unit mark indicates: 14 Fuss-Artillerie-Regiment, Bataillon II, [Munitionskolonne] Haubitzen 8, Waffe Nr. 6. The units formal name is: Badisches Fussartillerie-Regiment Nr. 14. Raised in 1893 with a home port of Strassburg (Still, correspondence). This translates as 14th Foot-Artillery Regiment, Second Battalion, Howitzer Munitions Column 8, weapon #6.
Foot artillery is Fortress and otherwise emplaced artillery. Unlike Field Artillery, German Foot Artillery units had little organic mobility and used heavy howitzers and siege mortars. The German Foot Artillery manned coastal defenses; but they also manned the guns of interior fortifications and were responsible for conducting set-piece sieges in the field if such an operation became necessary. The reduction of the Belgian forts along the Meuse River in 1914 was primarily a Foot Artillery operation. (Zabecki, "Steel Wind").
--Dwight