This tunic has the attributes of an M13 field uniform for the Beamten of the Kassenwesen (paymaster department). Everything looks pretty much as it should be. The collar patches (the colors are correct) are not the size I would have expected, nor are they positioned well . Normally, photos show them half that length and paralleling the bottom edge of the collar, just a centimeter or so above. The ones on this tunic are placed right in the center of the collar (from top to bottom) and almost disappear over the top of the collar. It looks a bit awkward and not what you would expect to see on an issue quality tunic like this. They look more the size of the elongated Litewka patches.
The rank is Beamtenstellvertreter. The military Zahlmeister officials had white metal buttons and members of the civilian (uniformed) Kassenwesen had gold colored buttons. This tunic was likely made in 1915, when the buttons were being transitioned to field gray painted steel for everyone. We can still tell from the gold color of the tresse on the shoulder straps that it is from the Kassenwesen (civilian). By September 21, 1915 the new regulations added an additional band of Tresse across the bottom of the strap. This helps to affirm the notion that this tunic dates from mid 1915 up to early 1916.
Regards,
Chip