Primarily, the makers mark is stamped into the crown of the helmet. In my experience, any painted regimental markings are also found in this location. The year date of any kind indicates the year the helmet was accepted into regimental stores. For the most part, this acceptance date, regimental number, battalion and kompanie are ink stamped onto the inside of the rear visor. Not all helmets have this complete sequence of information. By mid war, realizing that well marked pre war helmets contained a lot of information if picked up by the enemy the Germans stopped this practice. As a result, most M15’s have no markings except maker or date of manufacture. Some may have a Korps ink stamp in Roman numerals. Despite all of the above, you will encounter exceptions the longer you collect. You will also find prewar ink stamps sanded off or blacked out. Last , the Bavarians seem to have liked to sometimes imprint the regimental number into the leather. We have a list of regimental markings with photos here on the forum in the Pickelhaube Discussion area. In addition, we also have a list of manufacturers with photo examples.
