newbe: The marking you described indicates that the helmet was issued to Westphalian Infantry Regiment number 53. This regiment was part of the VII army corps and was garrisoned in Cologne-Deutz. The front plate on a helmet from I.R. 53 should be a standard Prussian line eagle.
A helmet issued to the Oldenburg Infantry Regiment should be marked "OJR 91". Infantry Regiment 91 was part of the X army corps and was garrisoned in Oldenburg. The front plate on a helmet from OJR 91 should be a Prussian line eagle with an Oldenburg wappen affixed to the chest of the eagle.
The article at the following link illustrates the unit mark for helmets issued to OJR 91.
http://www.pickelhauben.net/articles/Kammer_Marks.html
Reservist1
