Hello Bertrand:
It is not unusual for a helmet to not have the BAG mark. However, this is a Grenadier wappen as it has the S shaped guard on the sword rather than the"squared' guard on the typical garde wappen. My Didier Laine source indicates that the 3rd Garde Regts zu Fus would have this wappen BUT with Squared cross guard on the sword. So my first reaction is that the wappen is not correct. Perhaps other more knowledgeable members can account for this? Brian