Dorko is a fine german brand, and since it was established in 1920 there's not much more to go to date it more than that.
There is no model name (which sometimes help, sometimes don't), the 12 is not a model reference, probably more a blank reference (which would indicate that Dorko merely "finished" on a blank). But yeah, you can try asking Dorko.
As for the mill cutout I surmise it's merely a kind of spine decoration among many others.
There are several ways such a decoration can me made. It can be milled, but it can also be "stamped" in the red hot iron with pincers or with a file AFAIK.
The cutout often is lower on the blade, I just surmise that it's that high because they worked from a standard blank.
While it's not that common, it's not that rare a decoration either.
Here are two samples
https://i.servimg.com/u/f79/15/54/05/37/1828b210.jpg
This one is quite similar to your Dorko in termes of time of creation and provenance
Attachment 320985
this one is much older (~ 1880/90) and French made.