Nice M27. There should be an an other stamp in the steel in the top of the bowl. Might two letters and the two last digits of year of production, i.e. 31 or 32.
The chinstrap is not original to the helmet. It appears one end is a German buckle with strap. If the other buckle is a sliding one, it might be a civilian pre war strap, which were attached to privately sold M34's, with two sliding buckles, that was tampered with. Could be a war time adaption.
The military M27 (and M34 for that matter) chinstrap should be black with only one gripper buckle on the left side (on left cheek).